Messing with / customizing the brace may not be a good move: too likely ATF would view anything I did to it as creating a stock (length might get beyond the elbow or something weird when used as a brace.. and if it isn't USEFUL as a brace, they'd probably say its a stock). Besides, I'm not even sure how a mod to the brace could realistically be accomplished. There is specific wording in the letters about the braces to not modify them.
I don't mind getting something permanently attached at the muzzle to get the OAL up to snuff if really necessary. Sure, I'd love to keep the 3-lug usable to switch around devices. But the whole point is to avoid NFA stamps, so suppressors are unlikely.. and I can't see myself changing muzzle brakes / flash hiders all around anyhow. I mean. It's a 9mm. It's not like I need to experiment with more effective brakes or whatever. Find something that looks groovy and go with it. Granted, the bare 3-lug IS what looks the grooviest. But, whatever.
I still have to select which style of brace I want. Trying to balance lengths and looks. But I'm pretty sure that the B&T styled brace would get the stock SP5k to something like 23 and change inches. A slightly extended barrel, and a permanently attached MP5 grenade launcher device or longish linear compensator, and I'm good to go. There are 4" 9mm compensators. Sadly, they have flutes on the outer housing .. so it would look a little corny. I'm thinking I could find a place to make me a simple custom comp and have it look like a small suppressor. That might be too pricey though, and the HK grenade attachment would undoubtedly provide just as much blast/shock muffling as any linear comp. Plus its a legit HK part, at least.
A friend suggested using the linear comp and just making a sleeve to surround it, and then polish, refinish and mark it to look like a suppressor. Not a bad idea, actually.
As far as 922r goes, I am confident that would not apply. 922r begins with the phrase "It shall be unlawful for any person to assemble from imported parts any semiautomatic rifle or any shotgun which is identical to any rifle or shotgun prohibited from importation under ..."
922r deals with long guns only. Pistols are exempt.