Well, if it's a glock and you take the stock off then you can concealed carry it with a permit, a sbr isn't covered with a ccw. So if in non-sbr configuration it is no longer subject to NFA rules, then you should be able to treat it as a regular pistol. As far as someone else having it, it could just be in a vehicle in non-sbr configuration and the wife takes the vehicle to the store or it's in camp with my buddies and I head out for an afternoon hunt then it's not NFA item, no big deal.
I also like the idea of maybe a 300 blackout pdw pistol with red dot but be able to toss a stock and thermal scope on it and do some night hunting. All you have to do is take the scope and stock with it. It's a pistol while driving around but at camp or the cabin can put the accessories on to switch from car ccw to night hunting without hauling a bunch of guns or just being able to put a stock on when at the cabin or in camp when no longer in a ccw environment.