I've never heard of claiming from a show's organisers for damage done to a car while there. It could never be proved that it had happened at the show either

It's at the owner's risk taking a car to a show and especially as most shows are on private land.

If you caught someone yourself doing it, don't really see what you could do to *make* them pay, they are hardly likely gonna admit to it, so your word against theirs, plus they're more than likely to give a false name and address if you want to pursue it.

Just ensure on all advertising and/or tickets that it's at owners own risk.