As SkiAdcock said, even the most popular celeb restaurants are hit or miss. Famous folks may even be there, but you might not recognize them, or the lights will be low, or they will be in a more secluded part of the restaurant. Not sure how old you are, but going to a club like Shag (or whatever is happening that night) might be better for your purpose. I think the Concierge at the Bel Air should be able to get you on most guest lists.

Also, I assume you have the WB studio tour scheduled? I am not sure exactly where the tour goes, but everytime I go to the WB lot I see someone I recognize.