Another thought if you want to pursue a restricted mag release instead of a finned grip.
Why not use your standard mag release with a sheet metal guard with a small hole in it for a tool to release the mag? The guard could be held in place by one or more of the pins in the lower reciever. At the most you would need pins with a larger head on one side, alot less engineering than coming up with a ad hoc bullet button for a non standard AR lower.
If you look at the Kel Tec carbines offered by DD's Ranch that is exactly what they did. It also give those who travel out of state or plan to move in the future a option to covert to normal use while in NV.
About the lowers. You mentioned no stripped lowers, how about complete lowers? For California think A2 stock and finned grip. The main reason I would consider your Glock magged lowers is because of the box of pre banned mags I bought in earlier years and most other Ca buyers most likely have the same idea. You can not legaly use a mag with more than ten rounds with those bullet button type releases.That is why a restricted mag release is less desirable than a reguler operated release with a fixed stock and finned grip to me at least.
In the linked thread below the question came up about modding the bolt on some brands of uppers to use on your 9mm lowers. I have not noticed any difference in 9mm bolts which I thought were all "Colt" style except Olympic, do you recommend modifying the bolt on RRA uppers also?