If I had one of those I'd just put it away and dummy up. If you "register" it, it's gone. I certainly wouldn't modify a firearm to suit their current whims, if I wanted a firearm like they want, I would've built or bought it that way.
This will most likely go away, and if it doesn't, there's not going to be much "gun confiscation" going on when the shit hits the fan.
I'll tell you what, if you've got pistols that require a brace, you've got a lot of other good shit too. Go ahead and register the one gun. You'll be the go-to guy whenever the gov't gets a hair up its ass. When they come for that "one gun", they'll have to take the rest just to be sure they're "legal". You can take them to court to get "the rest" back.