What folks are saying is just because it is sold as a brace doesn't mean the arresting officer will agree. Otherwise, they would just call it a hoola hoop. I think the point is, even if you are correct ( and i don't think so) the cost to defend it will be a lot more than just buying a pre ban lower. Hell, even on a preban lower, I think you are going away and your firearm is getting confiscated while awaiting trial. Cops typically are not familiar with every aspect of the law, and you are not holding court on the side of the road.
Sure, you are probably going to win in the end. But it will come with a steep price. I think you are better off coming up with an alternate storage solution that will accommodate a pinned stock.