I'm not sure what you chaps are actually offering here beyond stating the obvious. I've provided links to Silent Custom and how they've done it - cutting into the gearbox and replacing it with a threaded insert. What I'm asking is if anyone knows of another way this is done. Clearly the answer in both of your cases is 'no', but it absolutely can and has been done by someone who now - for whatever reason - doesn't sell the adapters any longer/has dropped off the face of the earth. This is something only possible for HPA because there's spare room in the gearbox shell, so I expect that's why it's not so prevalent but give me some credit in knowing why this is a topic worth bringing up in the first place.
Thanks for the effort though. I'm assuming both of you have used GBBRs and can notice the difference in width between what I consider the wider and less comfortable AEG-compatible grips. Not to mention the lack of some of the variety in the real ones that one can freely use on near 1:1 receivers like the GBBRs do. That's why I want to do this, but I'm not sure why that matters because it's not what I'm asking.