Thanks Neanderthal for locating a legal perspective, it is definitely more informed than anything I could add. I am particularly interested in the difference between modifying an AR by adding the brace and purchasing an AR with the brace added by the manufacturer. I own a Sig P516 with the SB15 brace, and I want to be able to shoot it any way I see fit.
You can. This is just mental masturbation until ATF comes flat out and says they are a no-go if fired from the shoulder.