VCW

Hello @VCW,
welcome to this forum.
As mentioned above, there is no possibility to rename the reserve channel via a Macro.
There are some macros here in the forum to create luminance  masks.
I have created this macro, it generates a selection, a resserve channel and a greyscale layer.
The greyscale layer can be worked as you like and can be rasterised as a mask if necessary.
The disadvantage of the macro is that only one brightness is taken into account.
I had also put the macro together once, but the reserve channels were too confusing for me.
 
Cheers
Luminaz-Maske mit Auswahl, Graustufen-Ebene.afmacros

This guy is quite good re luminosity masks
 

Creating all those spare channels & deleting the extra layers is a lengthly complicated process ... so I wonder if anybody has tried making a macro to do that with a single click... 

Problem is you can't name spare channels in macros.

Bummer! Maybe someday we will be able to do that.

While firstdefense is correct, you can easily create and manipulate luminosity masks in Photo by other methods.
If you can explain what exact function you are looking for, we might be able to envision a workflow in Photo getting the same result, but utilizing the basic functions of Photo.

Welcome to the forum @VCW
Using Panels, Extensions, certain Plugins, Scripts and Actions etc designed for Adobe Photoshop CC, in Affinity, is not possible.

You could try using one or more of the Adjustment Layers: Levels, Brightness and Contrast, Shadows and Highlights, or Curves. Not necessary to use masks for that.

Choose your preferred search engine and enter 
    affinity editing landscape tutorial
 
you will get a wide range of tutorials.
Luminosity masks are not essential for this task.
you can use blend ranges as direct “replacement”, or create a luminosity mask if you like.
https://affinity.help/photo/English.lproj/pages/Layers/LayerMasks.html