I did car shows in the late 90's, and I don't recall any strict rules about what we could or could not wear, but then we didn't wear a uniform like they do at Hooters. Sometimes, depending on the venue, we wore particular outfits, but again, I don't remember any strict rules.
But all that worked to their advantage, because if a girl didn't understand sexy, she would never get a call back. So it creates a bunch of women competing to be "sexy enough." That meant that if there was no specified outfit, they felt the need to inform us of the decency laws of the state in which the show was taking place.
She says, she suggest wearing the skimpiest underwear you have or none at all ... the waitresses will tell you if your underwear is showing .... so it appears there is no rules as to what kind they may wear.