Yes, pretty much everything with a LA is allowed (it wouldn't be fun without an outsider, minotaur or vampire in the brawl). I also suggest we use the Gladiator rules from Dragon #303, with various archmages and avatars as the audience. So perhaps you should consider who'll be your teammates. (That will make it easier to set up the battles.)
PS.: This is a great opportunity to test all those nifty (and overpowered?) options in Unearthed Arcana
At the moment, I'm leaning heavily towards a Mind Mage (from a relatively recent Dragon; I can post it if you don't have it). Would that be acceptable?
Right now I'm thinking a 40th level fighter.
...With a permanent antimagic field item.
That or a void disciple.
Is everyone sure they want to be spellcasters ?
I don't have AU, Dieties and Demigods, or that Dungeon magazine but I do have the epic level handbook.