lizg- that really sucks. I've got my rental for 3 weeks, one before and one after the event. I don't think they realize where I'm going. I have heard of cancellations on people, so this may be their way of avoiding that. If you reserve without them knowing and get the damage waiver, it shouldn't be a problem unless it's truly thrashed.
I'm forgoing the insurance and using the coverage provided by discover card. I'm also being super cautious by covering everything with sheets, closing the vents once i get to empire, and leaving the car sitting for the two weeks i'm on the playa.
You might want to reserve without mentioning it. Say you're going to the Tahoe area. That's a somewhat truth!
Best of luck!