lacerto

Great trick, I will check it soon, thank you! This would solve my image editing problems. I'll solve the PDF back-testing with the Channels panel (like you did in Acrobat), and then I can actually use normal effects on the embedded images later.
Does Total Area Coverage, which is in Acrobat, also exist in Affinity? Or should I add up the percentages in my head?

You're welcome. You could also colorize K-only images, e.g., making them K100 using the Color panel (instead of being void of fill), and then applying swatch opacity, would equally retain the K-Only images as K-Only.

But doesn't it behave that way? Layer opacity does not cause a K-only image to become converted to CMYK. Layer adjustments (and most of the F/X blend modes) do:

K_opacity_and_adjustments.mp4  

As stated above, if you have Photo, you can use Channels panel and/or histogram. But you can cope with mere Publisher, as well, as you can use the Color panel to check color values of text objects (and vector objects), and adjustments like Channel Mixer to reveal color in parts that look black (K-only) but involve multiple channels.
 

poorishmanspreflight.mp4 As for causes Publisher will convert K-only to CMYK (multi-channel), there are various, three of which are mentioned in the video clip.
One further, very common reason, is applying any adjustment or blend mode on text or native K-only objects, causing rasterization to CMYK. Applying F/X effects will often also do that, even if outer effects and Gaussian Blur might survive non-rasterized. Using opacity percentage, too, would cause rasterization to CMYK in case  PDF output does not allow live transparency.
Last but not least, Affinity apps by default embed document ICC in a PDF export, which will cause incorrect readings in tools like Adobe Acrobat Pro in case "wrong" simulation profile is active, or when using Ghostscript for separation.

Note that you could export PDF/X-3 or PDF/X-4 based Grayscale PDF also from Affinity apps, but you need to have a true grayscale print profile installed on the system (I am not sure but on macOS you might have Generic Gray inherently available [UPDATE: Checked, and Generic Gray Profile is inherent on macOS, but is not available whenever using PDF/X-based method]; on Windows [UPDATE: and on macOS, too] you would need to fetch one from the Internet, or have one that comes with QuarkXP or Adobe available), since the only grayscale profile that comes with Affinity apps (Greyscale D50) is basically a display (Gamma) color profile and not selectable when you have a CMYK or Gray/8 document color mode and convert to Grayscale, and use PDF/X-based export method:
 
If you leave the profile empty (or do not have anything available), all the blacks defined above would be converted to four-color black and have Affinity app default U.S. Web Coated v2 marked as the output intent profile. If you do define a true grayscale color profile as output intent, RGB 0,0,0 and B:0 (Grayscale 0) will be output in ICC-dependent 100% Gray (CalGray/ICCBasedGray), but K100 will have something like a 92% value.
See below a PDF/X-3 with no explicit press-specific grayscale output intent (U.S. Web Coated v2 auto-assigned):
pdfx3_gray_from_cmyk_noexplicit_output_intent.pdf
...and a version where a true grayscale output intent is defined:
pdfx3_gray_from_cmyk_quark_genericgray_output_intent.pdf
UPDATE: The same applies to PDF/X-4 production (and in PDF/X-1a only CMYK color mode is available).
If you do not have to use PDF/X-based export method, but force the Grey color space, the default Greyscale D50 would be available, but then, too, K100 values would have non-100% gray values. So whenever forcing grayscale output, you need to have native color values (text and shapes) defined either in RGB or Grayscale, not CMYK.
There might be point in checking whether the printer truly needs PDF/X-based output in grayscale, or whether they just meant K-only output (but accept any regular non-PDF/X based CMYK-based output, as long as CMY plates are empty).
UPDATE: And if they just meant K-only, then the correct method to produce such PDFs is to choose CMYK document color mode and export to CMYK, and make sure that all native objects have K-only color definitions. There are plenty of other considerations that need to be checked when producing press PDFs from within Affinity apps, so the issues related to grayscale are only one of the concerns (but one of the most frequently experienced, and confusing, probably much because the very different K/Grayscale approach/policy used in Adobe apps within CMYK/press-related production).
In this context, paying Adobe for Acrobat Pro -- still also available as a 32-bit app on a perpetual license, though I am not sure how it is made to be operable on modern macs -- is not money wasted. pdfToolbox by callas software would be an alternative (still more expensive than Acrobat Pro). Free Packzview is not a prepress tool but is useful for decent preflight (it appears to be generally available only for professional use, though).

You cannot produce grayscale (8-bit) PDF/X-based PDFs using Affinity apps. All forms of color definitions (RGB, Grayscale, CMYK) will be converted to CMYK disregarding the target color mode selected at export time, whenever PDF/X-based export method is used. 
Technically PDF/X-3 and PDF/X-4 would allow ICC-based gray to be exported, and this is what happens if you do this from e.g. InDesign. But when done in Affinity apps, your definitions of gray and black will always be converted to rich black (four-color black).
You can produce grayscale PDFs based on Gray/8 document, but then you need to use non-PDF/X-based export method and explicitly export to Grayscale (and define your black/gray either using Grayscale or RGB color model; K100 will basically be converted to RGB and then presented as a converted CMYK value).
If your printer requires a PDF/X-based print PDF, you need to switch the document color mode to CMYK and then specify your blacks and grays using CMYK color model, and K-only values. The PDF output will be basically CMYK, with CMY 0 and with K-only values. Grayscale images that you place, will by default have "K-Only" button (available in context toolbar) applied, and will be exported as grayscale images. 

Thanks for the update. This was an instructive thread, because the job at hand appeared to be very simple, and what was confusing, was basically the very different way Adobe InDesign and Affinity apps handle "grayscale" (or basically, press-oriented black). InDesign does not basically offer "grayscale" color model at all, but uses instead Lab for specifically light intensity oriented color definitions, and for print intent documents always handles "gray" as K ink. So it is very straight forward. Whatever is defined in print intent document and using the factory default {Black] swatch (including tints) will end up on K plate when exporting to press, and stay "grayscale" in that sense. Similarly, imported grayscale images will aways stay on K plate, there is no need to explicitly tell that gray is not a composite RGB but needs to be treated as black ink.
What however was particularly confusing was bringing in PDF/X-1a and combining it with concept of grayscale, and specifically in context of InDesign, since when using this standard, conversion to grayscale proper is not available at all in InDesign (at least up to CS6), so to be able to produce specifically grayscale PDFs, anything else than PDF/X-1a needs to be chosen! PDF/X-1a is often asked for the purpose of guaranteeing that the job will be in CMYK-only color space and will not contain live transparencies, and is therefore a kind of fool-proof export method. 
So, to wrap up, what seemed to have happened was that as you had a pure b/w press-oriented job, you initially chose the Gray/8 document color mode and assumed that text and native art is safest to specify in  CMYK, using K ink only (as you would do in InDesign or Quark, where gray is not even available). Which in Affinity apps is wrong, since K would be handled as an RGB value in a grayscale document and would end up being either a four-color black or a diluted black (dark gray) depending on whether exporting to Grayscale or CMYK. So: in a Gray/8 color mode document, you need to define blacks (text and native art) that are intended to end up on K plate, as Grayscale or RGB color values, and make sure that you also export using Grayscale color mode (using CMYK at export time would convert grays to four-color black).  Placed grayscale images would also be converted to rich black. Using PDF/X-1a in context of a grayscale document, would work if you had press-oriented grayscale profile available (which Generic Gray Profile or D50 are not), so in this sense the color preflight warning that you would get trying to export using PDF/X-1a in context of a grayscale document, is correct. You will end up having four-color blacks if you use PDF/X-1-a:2003.
The next step you tried was probably switching the document color mode to CMYK/8. You already had your text and possible native art defined in K100 so it would seem obvious that you will have correct output having K-only color definitions and placed 8-bit TIFF grayscales. But no, your placed TIFFs would now be exported using four-color-black, as they would be treated as RGB images within Affinity apps whenever placed in Gray/8 or RGB color mode document, but handled in CMYK context, and would be exported as rich black when exporting to CMYK. Using PDF/X-1a would not change anything here.
At this stage, at latest, you might have heard an instruction from the printer to produce in true "grayscale", in the meaning: everything on one/black plate. The trick, in Affinity apps, is to apply the "K-Only" button on all placed grayscale images. This would not be done when placing these images in other than CMYK document color mode, and it won't be done automatically if switching the document color mode to CMYK. The button is also visible on the context toolbar only when using CMYK document color mode, and unfortunately you cannot apply "K-Only" for all document images at once, but need to go through images one by one and make the change. Note that when you have a document color mode in CMYK, placed grayscale images do have K-Only button automatically applied (and the mode will be retained also if you subsequently switch document color mode even if the button itself is hidden, which would also change the way these images are treated in these alternative document color modes when exporting to different color spaces).
After this, you would finally get what you need: "a simple grayscale" job where what you have defined in K-ink only and with imported TIFF grayscales, will be delivered as expected on one (K-plate) only (whether exported using PDF/X-1a or not). You should additionally be careful not to change color profile at export time, since that would, again, convert K-only colors to four-color values. [ALSO: It is good to remember that letting Affinity apps embed ICC profile in the document (= default), will produce an ICC-dependent PDF where using wrong simulation profile will show incorrect CMYK values in Adobe Acrobat Pro so there is still this additional confusion related to complexity of defining black ink values within Affinity apps.]
Part of this is related to clash of different technologies and underlying color engines, and terminology (some say "cultures", but I personally disagree), and the fact that you are not likely to get proper instructions from printers / print shops. The Affinity apps are also under development, and there seem to be recent changes e.g. in the way placed CMYK images are treated when they contain embedded profiles (nowadays they are discarded similarly as by default in InDesign). How could anyone expect to get step-by-step instructions from a third-party when they are not provided even by the first-party, practically ever? 

In lack of proper preflight tool, you could open your exported PDF in an Affinity app, and get pretty reliable interpretation of color mode of objects in the PDF (things like overprint status would not be read, at all, but you should be able to detect things like K100 vs. four-color-black).

These things are not impossible because something like this has been done already well over a decade ago 🙂

rtf_xlsx_links.mp4  

Ah, now that might work out, though it is a bit odd that the quality of the result relies so much on the blend mode being used on an effect.
Thanks again for your help  

Trying e.g. Lighten Blend Mode could be an acceptable workaround:

Lighten.mp4  
 
 

A lifetime of experimentation is how I got to where I am, so I appreciate it when others add their effort to mine.

That was a sad try as I later noticed that I had somehow managed to turn off scaling of all FX applied in the design myself, before turning them back on 🙂

No to that, too, so the release 2.1.0 continues to be broken. I am not sure if the state of brokenness is actually even acknowledged or if the change (which requires a deliberate act) is a new feature and that is here to stay.

The syntax is system region-dependent, so e.g. in Finland (and probably in Germany, too) the arguments need to be separated by semicolons instead of commas:
clamp(49;15;30) would give 30. [And if you need to specify decimals, the separator is also system region-dependent, so in Finland, and probably in Germany, a comma instead of period]
As for abs, as mentioned, it just returns the absolute of a value, so if you had e.g. negative x coord value -3 and you surround it with abs(), the x would be moved to positive value 3.

For Windows, Mac and Linux 
https://www.qoppa.com/pdfstudio/

I am not sure if it is 100% up to the etiquette, but as long as you're basically doing these kinds of things in purpose of helping users of Affinity apps (instead of trying to sell something, or mention the competition to harm Serif), it has been kind of ok to mention 3rd party tools (sometimes also competitive) that are useful when creating documents with Affinity apps.
Adobe trio very often gets mentioned on the forum because that's where many users of Affinity apps come from, and often expect (rightly or wrongly) identical results when using similar tools, operations and workflows. It benefits the users of Affinity apps to understand the differences and describe, often with direct comparison with an Adobe app, what they need to do to get expected results.
Personally I think it is a clear forte of this forum to allow at times blunt comparisons and even harsh critique against the Affinity apps. They have so many strong points (often superseding the competition), and they are in continuous development, that it is definitely worth to stay persistent and learn to use these apps thoroughly -- sometimes it means, learn to use them "properly" (in a different way that they were designed to operate), sometimes learning workarounds (mostly easy ones but sometimes tedious).

Personally I would do this kind of job by using a PDF editor and merging press-prepared documents, just replacing specific pages (page numbers saved as a list) and saving the opened CMYK PDF with a new name, meaning that the resulting file will have identical properties, but just have specific pages replaced with grayscale versions.
One such tool is PDF-Tools. In the following the replacing file and page ranges have already been defined into a custom batch job, which also changes the initial view to scrolling facing pages with a cover page and sets the magnification option to fit page:

mixed_pubs_xternal_tool.mp4  Using a PDF editor has the benefit of not needing to play with page positioning, print marks and export settings because these things have already been done in the source files.   

I do not see any other way than what I showed in my post, but that, too, may require some preparation (like switching between K100 <-> Gray/RGB black), and handling of placed PDFs (= need to use Interpret mode to have their color space changed to RGB and Grayscale). and also post-processing, like manually changing page boxes from trim to media/bleed page by page, and handling non-symmetric bleeds (which Affinity apps cannot deal with automatically).
The video clip below shows the procedure of creating a mixed mode publication, after you have created your all CMYK and and all Grayscale versions. The building blocks are attached, including an RGB version that has black text as RGB 0, 0, 0.

mixed_colors.mp4   But basically the method shown could be used pretty effectively to get fully professional output. Hopefully Affinity apps at some point will have InDesign [Black] intelligence which keeps K values in grayscale exports, and auto-converts K100 to RGB 0, 0, 0 in RGB exports -- and that Publisher has its page box bugs and usability issues related to the page box and K-Only button fixed -- these fixes would make creating mixed-mode print jobs very smooth.
color_and_gray2_allcmyk.pdf
color_and_gray2_allrgb.pdf
color_and_gray2_allgray.pdf
color_and_gray2_mixed.pdf

Thanks for demonstrating this new feature! This appears to be a genuinely new ((if not unique) concept and while it appears to be pretty resource-heavy, I must say I was impressed by the implications and the promise, also considering PDF 2.0. Maybe we are on verge of experiencing something truly new in multi-purposing and creating alternative PDFs -- my brain hurts already, and I sincerely welcome the pain caused by having to expand a bit my (age-old) professional universe  🙂 

Using adjustments only results in having four-color-gray on "grayscale" pages, so if you print commercially, this is not a solution (at least if your goal is to save in printing costs).
I would do this as follows:
Prepare your color document in CMYK mode. You can use RGB definitions for native shapes and colorful text if you want to benefit from the full RGB color gamut. Specify all black text that needs to be in K-only using CMYK color model, and CMY set to 0. Export your RGB document. [OR: If you want your K100 blacks as RGB 0,0,0, wait until you have converted K blacks to RGB blacks.] Export as "All pages" a fully colorful CMYK document using the document CMYK profile. If you have bleeds, include them but do not include any print marks. If you have placed PDF in color, mark them to be exported as "Interpreted" (instead of "Passthrough"), since in Affinity environment color space of the "passthrough" PDFs is retained (because Affinity cannot perform that kind of deep modifications in placed content, similarly as e.g. InDesign or QuarkXPress). If you want to have K100 text on the grayscale pages to become K100 (instead of dark gray having something like 87% blackness)m change the color definition of K100 text to RGB 0, 0, 0 or Grayscale 0 (they are practically a same thing within Affinity apps). You can do this in one go using the format option of Find Replace tool. Now export "All pages" by specifying explicitly "Grayscale" as export color mode and choose "Grayscale D50" as the color profile (if you do not have anything more appropriate available). Here, too, if you have bleeds, include them but do not include print marks. Prepare an empty Publisher document that uses the same format and CMYK color profile as your original CMYK publication. Now place the grayscale version of your PDF in this document, expand the placed content suggestion on the left column so that you have all pages shown in the list of pages to be placed, then select them all, and on the empty page of the hosting document, click in the exact middle of the page of the hosting document, or draw a page rectangle of the size of the document (including the bleed, if applicable) on the hosting document so that all pages from the grayscale PDF version are placed in the new document. If you have bleeds, change the Page box for each imported page so that page crop is set correctly to include bleeds. If you do not have bleeds, then the default "Trim" crop option is correct. Publisher is buggy at the moment and might have placed the pages inaccurately (specifically if you just clicked in the middle of the page instead of having drawn the page rectangle) so you need to check that placed pages have been positioned correctly.  Now that you have grayscale versions of all pages placed, use the Resource Manager, and for each page that you want to have in full color, replace them one by one with the all CMYK version of the publication. Publisher will automatically fetch the correct destination page from the replacing PDF so if you select page 4 in Resource Manager and replace that with the all CMYK version, page 4 from that document will be chosen. Now export the document using the same CMYK settings using PDF (press ready) preset, which is the most flexible export method and will not rasterize any of your placed PDF content. Do not include ICC profiles because all your CMYK pages will be DeviceCMYK, and all your grayscale pages "DeviceGray". Attached are PDFs of a simplified project. The placed content has RGB image, RGB Photoshop image with transparency, placed CMYK PDF, native RGB shapes, and K100 text placed on each of the four pages.
color_and_gray_allcmyk.pdf
color_and_gray_allgrayscale.pdf
color_and_gray_mixed.pdf
The weak point of this method is needing to have PDFs with color content set in Interpretation mode since that might result in e.g. font replacements and other interpretation errors. That would not be a problem in the referred "other" application, at least if your "other" is the "other" of mine.
Another possible weak point is if your printer requires e.g. PDF/X-based preset method, since Affinity apps have a thing called "PDF compatibility rules" which e.g. means that a non-compatible placed PDF content will be rasterized. And rasterization results in having four-color conversion and accordingly ruins the grayscale plans.

Hi @jdc5294,
Just to throw an alternative approach into the ring...
The Affinity Suite introduced Layer States in version 2.4.0 released earlier this week. This would give you the ability to set your file up and switch between colour, greyscale or a combination of the two using two mouse clicks all within the same Publisher document and export accordingly for both the online and print versions...
Creatre a CMYK document and set your text and line elements using K100 and include the appropriate versions for image and graphic content based on the final output required which will be either RGB, CMYK or Greyscale.
You can then set up Layer Queries using Layer Tags, Layer Types or Layer Names defined using Regular Expressions all of which can be used to determine which layers are shown or hidden for different versions of your document.
Layer Queries Set For Different Output Destinations in Publisher

Layer States.mp4 Output From the Three Different Layer States Shown Above

Separations.mp4 This is a link from the Affinity Photo Help file which provides an overview of Layer States and Queries as it doesn't appear possible to link directly to the Affinity Publisher Help File for Layer States but the information applies equally across all three apps...
https://affinity.help/photo2/en-US.lproj/index.html?page=pages/LayerOperations/layerStates.html&title=Layer states

Using adjustments only results in having four-color-gray on "grayscale" pages, so if you print commercially, this is not a solution (at least if your goal is to save in printing costs).
I would do this as follows:
Prepare your color document in CMYK mode. You can use RGB definitions for native shapes and colorful text if you want to benefit from the full RGB color gamut. Specify all black text that needs to be in K-only using CMYK color model, and CMY set to 0. Export your RGB document. [OR: If you want your K100 blacks as RGB 0,0,0, wait until you have converted K blacks to RGB blacks.] Export as "All pages" a fully colorful CMYK document using the document CMYK profile. If you have bleeds, include them but do not include any print marks. If you have placed PDF in color, mark them to be exported as "Interpreted" (instead of "Passthrough"), since in Affinity environment color space of the "passthrough" PDFs is retained (because Affinity cannot perform that kind of deep modifications in placed content, similarly as e.g. InDesign or QuarkXPress). If you want to have K100 text on the grayscale pages to become K100 (instead of dark gray having something like 87% blackness)m change the color definition of K100 text to RGB 0, 0, 0 or Grayscale 0 (they are practically a same thing within Affinity apps). You can do this in one go using the format option of Find Replace tool. Now export "All pages" by specifying explicitly "Grayscale" as export color mode and choose "Grayscale D50" as the color profile (if you do not have anything more appropriate available). Here, too, if you have bleeds, include them but do not include print marks. Prepare an empty Publisher document that uses the same format and CMYK color profile as your original CMYK publication. Now place the grayscale version of your PDF in this document, expand the placed content suggestion on the left column so that you have all pages shown in the list of pages to be placed, then select them all, and on the empty page of the hosting document, click in the exact middle of the page of the hosting document, or draw a page rectangle of the size of the document (including the bleed, if applicable) on the hosting document so that all pages from the grayscale PDF version are placed in the new document. If you have bleeds, change the Page box for each imported page so that page crop is set correctly to include bleeds. If you do not have bleeds, then the default "Trim" crop option is correct. Publisher is buggy at the moment and might have placed the pages inaccurately (specifically if you just clicked in the middle of the page instead of having drawn the page rectangle) so you need to check that placed pages have been positioned correctly.  Now that you have grayscale versions of all pages placed, use the Resource Manager, and for each page that you want to have in full color, replace them one by one with the all CMYK version of the publication. Publisher will automatically fetch the correct destination page from the replacing PDF so if you select page 4 in Resource Manager and replace that with the all CMYK version, page 4 from that document will be chosen. Now export the document using the same CMYK settings using PDF (press ready) preset, which is the most flexible export method and will not rasterize any of your placed PDF content. Do not include ICC profiles because all your CMYK pages will be DeviceCMYK, and all your grayscale pages "DeviceGray". Attached are PDFs of a simplified project. The placed content has RGB image, RGB Photoshop image with transparency, placed CMYK PDF, native RGB shapes, and K100 text placed on each of the four pages.
color_and_gray_allcmyk.pdf
color_and_gray_allgrayscale.pdf
color_and_gray_mixed.pdf
The weak point of this method is needing to have PDFs with color content set in Interpretation mode since that might result in e.g. font replacements and other interpretation errors. That would not be a problem in the referred "other" application, at least if your "other" is the "other" of mine.
Another possible weak point is if your printer requires e.g. PDF/X-based preset method, since Affinity apps have a thing called "PDF compatibility rules" which e.g. means that a non-compatible placed PDF content will be rasterized. And rasterization results in having four-color conversion and accordingly ruins the grayscale plans.

Using adjustments only results in having four-color-gray on "grayscale" pages, so if you print commercially, this is not a solution (at least if your goal is to save in printing costs).
I would do this as follows:
Prepare your color document in CMYK mode. You can use RGB definitions for native shapes and colorful text if you want to benefit from the full RGB color gamut. Specify all black text that needs to be in K-only using CMYK color model, and CMY set to 0. Export your RGB document. [OR: If you want your K100 blacks as RGB 0,0,0, wait until you have converted K blacks to RGB blacks.] Export as "All pages" a fully colorful CMYK document using the document CMYK profile. If you have bleeds, include them but do not include any print marks. If you have placed PDF in color, mark them to be exported as "Interpreted" (instead of "Passthrough"), since in Affinity environment color space of the "passthrough" PDFs is retained (because Affinity cannot perform that kind of deep modifications in placed content, similarly as e.g. InDesign or QuarkXPress). If you want to have K100 text on the grayscale pages to become K100 (instead of dark gray having something like 87% blackness)m change the color definition of K100 text to RGB 0, 0, 0 or Grayscale 0 (they are practically a same thing within Affinity apps). You can do this in one go using the format option of Find Replace tool. Now export "All pages" by specifying explicitly "Grayscale" as export color mode and choose "Grayscale D50" as the color profile (if you do not have anything more appropriate available). Here, too, if you have bleeds, include them but do not include print marks. Prepare an empty Publisher document that uses the same format and CMYK color profile as your original CMYK publication. Now place the grayscale version of your PDF in this document, expand the placed content suggestion on the left column so that you have all pages shown in the list of pages to be placed, then select them all, and on the empty page of the hosting document, click in the exact middle of the page of the hosting document, or draw a page rectangle of the size of the document (including the bleed, if applicable) on the hosting document so that all pages from the grayscale PDF version are placed in the new document. If you have bleeds, change the Page box for each imported page so that page crop is set correctly to include bleeds. If you do not have bleeds, then the default "Trim" crop option is correct. Publisher is buggy at the moment and might have placed the pages inaccurately (specifically if you just clicked in the middle of the page instead of having drawn the page rectangle) so you need to check that placed pages have been positioned correctly.  Now that you have grayscale versions of all pages placed, use the Resource Manager, and for each page that you want to have in full color, replace them one by one with the all CMYK version of the publication. Publisher will automatically fetch the correct destination page from the replacing PDF so if you select page 4 in Resource Manager and replace that with the all CMYK version, page 4 from that document will be chosen. Now export the document using the same CMYK settings using PDF (press ready) preset, which is the most flexible export method and will not rasterize any of your placed PDF content. Do not include ICC profiles because all your CMYK pages will be DeviceCMYK, and all your grayscale pages "DeviceGray". Attached are PDFs of a simplified project. The placed content has RGB image, RGB Photoshop image with transparency, placed CMYK PDF, native RGB shapes, and K100 text placed on each of the four pages.
color_and_gray_allcmyk.pdf
color_and_gray_allgrayscale.pdf
color_and_gray_mixed.pdf
The weak point of this method is needing to have PDFs with color content set in Interpretation mode since that might result in e.g. font replacements and other interpretation errors. That would not be a problem in the referred "other" application, at least if your "other" is the "other" of mine.
Another possible weak point is if your printer requires e.g. PDF/X-based preset method, since Affinity apps have a thing called "PDF compatibility rules" which e.g. means that a non-compatible placed PDF content will be rasterized. And rasterization results in having four-color conversion and accordingly ruins the grayscale plans.

Thanks @lacerto, yes as anyone who's worked with printers before knows, "black" can mean many different things. I'll fool around with both methods this weekend on the free trial, I had a hunch an adjustment layer wouldn't produce actual "black".