Classes from the event flyer:
AS, ASR, GT1, GT2, GT3, SPO, SPU, T1, T2, Spec Miata, Stock Cars, Specials, Vintage, All Showroom Stock, Spec Miata, BP, DP, EP, FP, GP, HP, All Street Prepared, All Improved Touring, Street Modified, CS2, CSR, DSR, S2, SRF, GTL, CF, CFC, CF , FA, FC, FF, FM, FS, FV, F500 and RALLY (must conform to Rally America 2011 rules).
I think it's pretty much SPO or nothing, unless it's listed in a Street Prepared class (which is a Solo I rule set, not road racing, so I don't know much about it anymore).
K