It's a good discussion and personally I believe what Troy said. For practical reasons the conclusion I've come to is for all practical purposes the vast majority will not have to test this law, and if so, can base the worst outcome as confiscation.
Most of us don't plan on using our firearms in the commission of felonious acts, if so all bets are off.
So, how will they find out? Let's say you have a house fire and a fireman finds your PWA in the closet, he hands it over to a policeman. The bored policeman "runs" it sees it could be "evil from the dark side." So he turns it over to the local DA or whatever. What's the case, you can't prove it is pre-ban but certainly could be. They can't prove it's not, and don't have to so it's 60 -40 their favor. Since that is the only charge they have on you I don't think they would pursue it. A jury could just say no for several reasons. I think they would pressure you to give it up to avoid charges. That's a sure win for them, at the least amount of effort and cost.
Now, if you were cooking crack in your garage that starts the fire, you're on your own!