I know more about Moxie's, Joey's, etc than most people (many of my university friends worked there). They were pretty forward about what happens. They know they were hired because of their looks, one purposely chose to show her cleavage because she noticed tips go up depending on how low slung the shirt is...
They also told me the restaurants hire one token 'ugly' girl in case of a human rights contravention.
That is certainly wrong, but how do you police it?
I guess the easiest way is that you don't and wait for someone with enough guts to challenge, then you prosecute. Not exactly pro-active or preventative, but maybe the goal here is to ensure there is a recourse for those who does challenge the wrong? Awful, but it will probably be something that end up written into the expected expenses of the restaurant.