The difficulty is, and what most folks willfully ignore, is that they won't go at it that way. They know they can't win that, so they will make you look like a weasel. It is, was, and always will be about intent with the ATF.
"Ladies and gentlemen of the jury, clearly this man bought this "pistol brace" to circumvent the NFA and to avoid registering and paying his $200 tax. Please allow me to admit into evidence some of his postings from a website called theoutdoorstrader.com where he clearly shows willful disdain for not only the government but the homosexual community, blacks, and women."
Go ahead, shoulder that brace. No one's been arrested......yet.
Do you mean that perfectly legal brace? It is legal to own, and legal to have on the pistol. Proving that someone actually shouldered it during a home invasion (hypothetical), especially when they deny it, is going to be a tough one to prosecute.
I'm not advocating doing things illegally, but unless the goons actually catch you with irrefutable proof of breaking the law, they have no case.
Some of the ODT postings could hurt.
It's a ridiculous law, anyway.