I'm personally the biggest fan of the GT Sprints classing system - brackets. Set a laptime range for each class and have the drivers pick one (with their final time deciding which class they get placed in). I know this leaves it open to sandbagging but I've never heard of it being a problem with CASC. You could always add a God clause where if you feel someone is sandbagging, like lifting on the straights, you disqualify them or they automatically go in GT1.
This way, the cars don't matter. I think it's especially good for an event like this, where a lot of novices will be competing, some with cars that are faster than they are (like me, I know my car is capable of more than I'm prepared to risk achieving). This way someone with a tuned Civic who's an expert on the track running 20s wouldn't compete against a newbie in a 911 GT3 who coasts through corners.