I ended up wearing a jean skirt and polo with t-shirt underneath. It was unGodly hot, but it worked. They really weren't as concerned with the specifics of the outfit as the overall look. And if someone had something on that obviously wasn't from the 90s they just made them turn it around, turn it inside out or wear one of the radio station t-shirts they were giving away.
It was a looong day (7:00 am - 8:00 pm) and I didn't win anything (they were having drawings of memorabilia, t-shirts and even two ipod nanos for the extras) but it was really interesting to see how scenes and shots are set up and to learn more about the movie industry. And most of the people there were really cool and down-to-earth. I could go back today if I wanted, but I think I've had enough of being a star.