I've been following this story quite closely over the past week and it seems like there is really no consensus on the interpretation of the letter released just before SHOT Show but a very clear interpretation of the two private letters that came before it. Of course, the speculation that the ATF acted on the most recent letter is because of the overwhelming applications they are getting for new devices seems accurate to me. I've seen all sorts of devices, some even for short barreled shotguns. Clearly, the creator of the brace device was not aware that short smoothbore ANYTHING is an AOW per the ATF already.
My take from all of this after the smoke clears is that there had to be some sort of response to the outpouring of new devices and to address the rising popularity of the Sig Arms Brace. People are rabidly building what would otherwise be SBRs with the intention of creating a legal SBR without the restrictions of NFA. Your "Get Out Of Jail" card is that your "intention" was to manufacture a pistol and use the brace as intended. I did not take the letter to interpret with a blanket statement that shouldering a pistol or any attached device would constitute an SBR, just that you should be warned against manufacturing an SBR that you intend to shoulder. The ATF's letter is telling you that if you just intend on getting around the tax stamp, you are in violation of the NFA. How they will establish your intent is, quite fortunately for you, up to them to prove.
They will not be hauling people off to court over shouldering a brace-equipped pistol but it would be a smart move not to poke the bear on the issue. After all, they won't have to enforce it as no public range with a range officer will likely let you shoulder any pistol after this letter. They wouldn't allow it because they are either ill-informed or it is a liability issue. If you build a brace pistol and never talk about your intentions with it, then I don't see any laws being broken. There is no law against the misuse of your firearm whether it be shooting it sideways, upside down, or braced against a body part or piece of furniture. For the unfortunate few of us who don't have the time or facilities to shoot our guns that much anyways, the letter doesn't change anything. If you wanted to build one to show off at the range, you may want to look into something else.
If you live in Illinois like I do, you have even less to fear. Generally, gun crimes are never charged against victims of legitimate self defense within a domicile. There is probably numerous case law supporting this, but I can recall that there is a specific court case in the eighties or nineties where a FOID violator was not charged at the behest of the Attorney General and the precedent set. I'm not a lawyer but my advice is free. Build your pistol, enjoy it, and for Kris Kringle's sake, don't post on Facebook about it!