The reason I am cautious is, that the law has been that way for years in MA. I figure someone in an effort to sell more ARs would have done it already. I am guessing it is cheaper for MFRs to sell "compliant" ARs without having to machine off bayo lugs and weld on muzzle brakes. Just attach free state uppers to CA lowers, and done deal. Furthermore, since FFL dealers can posess non compliant rifles, they could have in effect purchase cheaper rifles, and add bullet buttons.
The reason I am interested in this is that there is a NH gun shop that always seems to have great deals on ARs, even in FR mode, even when no one else had any, but they were never MA compliant. At the height of the madness, they said their gunsmith was too busy to "massify." This seems like a 5 minute swap out at best. So I guess, could the NH dealer be convinced to do this conversion, if you showed up with the parts?