As far as availability or legality of the stripped lower receiver, my understanding is that they are LEGAL and unaffected by SB23. In order to be an assault rifle, aside from the list of Make/Model specific (no "AR-15"'s etc) banned firearms, it must have a detachable mag and at least one other "evil" feature. A stripped lower has no pistol grip, so it is not an assault weapon. Assembly of the receiver with a pistol grip would be illegal, as would assembly in any other configuration that would include the detachable mag and any other "evil" feature; bayo lug, flash hider, collapsable/folding stock, pistol grip, etc. You would not be eligible to assemble it and then register, as it would be "manufacturing" and assault weapon after 1/1/2000. As for the rimfire/centerfire issue, that's something I had not thought much about. I would assume, as long as the lower receiver wasn't stamped with a caliber designation, and you were using a dedicated rimfire upper (as oppesed to a 223-22lr conversion), it would be legally a rimfire rifle, and as such, not affected by SB23. This is my guess, and opinion, and won't mean anything in court, so don't take it for fact. Call DOJ (maybe even the sherifs dept.) and see what they say about it. Good luck.
-John