Rifles with barrels less than 16" in length or with an overall length less than 26" are strictly regulated by federal law under the National Firearms Act (NFA).
Telescoping and folding stock rifles are measured for the 26 inch overall length with the stock in the fully extended position (not folded, not telescoped closed). Leave the barrel alone and you're good to go. Other than cobbling the 15-22 lower receiver.
I believe the lower receiver extension has an internal steel tube in it so you're in unexplored territory. I'd wait until one of the accessory makers, or S&W themselves develops a folding stock. Perhaps the M&P15-22 pistol due out in April is a good start. At least the lower receiver is already short.
You can not legally make a pistol out of a rifle, so forget that. You can, however, put a stock and 16" barrel on a pistol and be legal. And swap back and forth. Can't end up with a stock on a 7" barrel, but a stock-less 16" pistol is legal but hardly practical. Again, it has to be a pistol, not a "sawed off" rifle. The M&P 15-22 is only about 23" OAL if you cut the stock off at the end of the receiver and will not meet the 26" minimum OAL unless there's a stock on it, and you can't legally make a pistol out of it.