So that girl was the assistant of the assistant director ihihi!!
I wish I know who's her voice actress so I will add her page!! By the way can you answer to my question in your talk page (Scooby-Doo Stage Fright characters)?
In the credits there are Nancy; Colette; Donna; Cathy; Amy : four of them are part of the Girl-O-SaurusRex. One of them is Cathy. So the assistant can be Amy Nancy Colette or Donna. I have to buy the DVD so in the subtitles maybe they'll show characters's names!