Full-tread (or shaved to 6/32) Toyo RA1s have generally been the poor man's alternative to Ho-Hos. Not as fast, but won't over-heat if the track dries out either.

That said, as one who has thus far resisted the urge to spend major coin on a set of tires I may use once a season, I generally just avoid racing the the rain whenever I can.

(Ironically, my only SCCA class win came last year on a track that was wet when we started the race, and was still damp with some puddles when we took the checker. Go figure...)