You're probably right, ShooterPatriot. At least about a welded-on extension being acceptable. Whether the gun turns back into a handgun or remains (legally speaking) a "short barreled rifle with no stock" when it's removed from the chassis is an ongoing debate. I'm of the opinion that it remains a rifle (again, purely from a legal standpoint. And that doesn't mean I agree with it) but that's been discussed in other threads.
The more I think about the extension the more I believe that it would be acceptable as long as it met the ATF's requirements for being permanently installed.
I'm not sure that I'm going to go this route, though. I'm sure that a Micro Roni isn't the most practical SBR or the most value-added use of SBR funds. I just think they're cool. Therefore I want one. Plus, I already have a Glock and a lot of mags. But welding a 3" extension onto it begins to lower it's "cool factor" enough that its limitations begin to overshadow its appeal, IMO. Thanks for the advice on the MP5s, GeneralPurpose. I've always admired those. I just wish that Illinois didn't have the 26" OAL limit. The way the law reads to me, <26" should be allowed but I guess the ATF is erring on the side of restriction. But we couldn't have SBRs at all until recently so I guess I'll be happy with that for now.