Check out this topic on WERA board, it's been discussed many times.
Some people got away by telling the insurance company only the location of the accident without mentioning the track event. Some insurance only decline the claim if it's a "timed" event so track day was covered.
But most people there have the mentality of you pay to play.
I personally don't think it's worth to worry about this so I just got myself a race prepped used bike and be happy on the track
