Great idea Amp
However, if you want to make this a real life competition, you have to think about it in terms of what pay site designers do in real life. What happens is a client comes by and says : "This is my domain, and this is the content set I want to be used for my paysite design". Then the designer does a paysite design using THAT domain and THAT content, since those are the rules of real life graphic design.
Therefore if you want to really measure designers' skills, you have to give them 1 domain and 1 content set to work with. This way the rules will be closest to real life and you'll really see who is a good designer and who knows how to do just one specific thing. Cause think about it - when a customer comes by and says I want this teen site done, do you tell them "Well, I prefer doing mature sites, so sorry, but you have to look for somebody else"
This should be a real life competition, and if somebody wants to let loose their creativity and work with whatever they feel most comfortable, they should go draw a painting or something.
Just my
Cheers,
The Machine