Hi everyone. I've looked over many of the posts on this thread in between helping out a friend who needed to figure out how to designate an 8 bit greyscale image as a layer's transparency. I'll have to admit that I haven't read through every post, but I read all of the 1st page, most of the 2nd, then jumped to the last page of posts. It really was a strange little adventure finding a solution. The weirdest part of our experience (we did a couple of zoom sessions so that I could share his screen, etc.) was when I found a solution, had him duplicate exactly my steps and lo and behold it didn't work on his machine!. The biggest difference between our boxes was that he had a later operating system version than mine (mine: Mojave; his: Big Sur). That first attempt (which, again, is a solution for me, but not for him !???) was a variation on the spare channel idea, but, since my 2nd solution was simpler and worked for both of us, I'll ignore that first one here.
Just to reiterate what the desire was:
He had an rgb image document (the character with a background that needed removing) and a separate document which had alpha values (a greyscale image - in this case, what we used to call a "window matte" in which the black was the "background" - ultimately used to determine the completely transparent part of the layer - and the 100% white was the purely opaque part).
Copy the layer from the greyscale image and paste it as a layer into the document with the rgb layer. Now you have, in the layers palette, two layers, rgb on bottom, greyscale alpha on top;
Select the top layer (click on left side layer icon);
Go to the Filters menu and select Colours>Erase White Paper. (suddenly what you see is the rgb character surrounded by black, because the upper layer has had its white pixels turned to transparent);
Command-click on the (top, now only non-white values) layer (on left side layer icon) to load that layer's transparency as a selection (this is the same age-old functionality as Photoshop);
Turn this layer off and select the rgb layer below (left side icon again duh);
Background now transparent.
This can also be recorded as a macro, of course. I like the way Affinity's macro recording works (I've only just started playing with Affinity).
My friend thought he was going crazy trying to find out how to do this. I told him that he was NOT crazy at all because there is nothing intuitive about this, in spite of the fact that Affinity is obviously a very deep and powerful tool. It's ridiculous that at one point (when I was outside away from my machine) I said "well what if I just write a script that puts the greyscale from one image into the alpha channel of an rgb (or rgba) image and I can send it to you?" and we both laughed because there was just no WAY that Afffinity couldn't do this. Then I told him, after we found the 1st "solution" that failed on his machine, that he could just keep sending me files and I could fix them and charge him like a thousand bucks per file and we both laughed again, but me more than him.
Anyway, I hope this helps someone.