Well, if we don't know, we don't know.
Anyway, try State Farm. No personal experience with them, but I've heard some agents are cool with it (being local autocrossers themselves...)
Otherwise, most companies will not cover you during these types of events, and you are lucky if they don't drop you outright once they find out that you ever do this type of thing. (Nevermind that it makes you a better driver for the 99.5% of the time that you are not autocrossing.) Stupid bean-counting actuarials...
