I think the overriding question would be, could this be construed to be a buttstock?
I understand one side of the
argument...
But the ATF can be fickle... On an AR pistol, the buffer tube is a necessary part of the action and the Sig brace is just a brace that is added to it. But, on a shotgun, there is no need for a buffer tube so the only reason you'd put one on it would be to attach the brace. Who knows how that would be viewed. If it were to be viewed the "wrong" way, you could be charged with having an unregistered SBS. And even if you were ultimately correct, and your trial showed you were within the law, is it worth the hassle? Just because you may be technically correct that doesn't mean that your local LEO and DA will see it your way. Tax stamps are cheaper than attorneys....
Curious idea, but not worth the worry of potential hassles for me. Not just to save a few bucks.
Personally, at ~$140 for the stock, I'd rather play it safe and just kick in a few extra bucks and get a stamp for a SBS. But that's just me....