A small update: A couple of my friends decided to get on this crazy train, and one of them wanted to see if an Archangel stock and the corresponding magazines could be made to work. It appears that the factory Archangel mags can in fact function fine with 8.6 Blackout (or any other .308 based cartridge hypothetically) by simply replacing the little wire "feed lip" on the magazine, without any other modification being needed.
We bent the prototype part out of some welding wire we had laying around. This proved the concept, but the wire is too soft to be functional long term. Some music wire has been ordered to make more a more reliable part. It's actually super easy to disassemble the Archangel mags to make the swap, too.
The Archangel stock is not my cup of tea. But it's hard to deny the functionality it offers, so I understand why one might choose this. My build will still use a wooden stock, but it is kinda cool that the Archangel is so adaptable.