"It only matters if you get caught."
True with any violation of the law. But the trouble with even those "minor" kinds of violations, it only takes one zealous ATF agent, eager for promotion, and a U.S. Attorney who hates gunowners and wants to impress his boss, and some poor slob ends up in jail for years. Every one of those minor "crimes" is a federal felony and folks can not only go to prison but lose the right to own any gun forever.
Not really worth it just to play with a forbidden "toy".
FWIW, I have a stocked Red 9, and had a stocked BHP. I have fired stocked 7.63 Mausers and .45 pistols and other handguns with stocks. IMHO, they are all much overrated, not a lot steadier or more accurate than a simple two-hand hold. One big drawback, except on a range, is that the pistol muzzle ends up only a few inches from the left ear, and the blast from a 9mm, 7.63mm or .45 is pretty bad without hearing protection. The Broomhandle is marginally better because of the longer stock and barrel, but it is still bad.