That would be great if our laws were built on common sense.
It may be dumb, but not breaking the law isn't hard.
It's no more difficult to keep a brace on a pistol off your shoulder than it is to keep a stock off your pistol and off your shoulder.
So use it like a brace and no problem.
Do what you want and chances are there will never be a problem. I don't prefer to gamble on the 1% chance (or whatever it is) of a criminal charge and I would never counsel someone that there is no legal issue here. The letter is information about how the ATF intends to enforce the law. There are no guarantees that a judge or jury would see it their way, but there is no guanrantee they will see it another way either.
Let's get the law regarding SBRs and SRSs changed and forget about this nonsense.
Treat the disease and stop monkeying about with the symptoms. Good advice.