I'd e-mail one of the UFC talent directors through their corporate website. Your best bet is to also get in with some trainers/promoters who the UFC trust to tell them about up and coming talent. Being a ton of people try out for the show yearly it would greatly help your chances if someone like that recommended you to them and prior to the cattle call tryouts. Each year on the show you usually hear Dana say certain guys are contenders to win it because where they were training or because he had heard good things about there fighting. That would tend to make me think more guys are picked for the show in advance than end up making it just by the cattle call audition process.
Odds are they don't have a perfect rotation of what weight classes they will pursue for the show. I would guess they look at what weight classes they need to add more depth too. Plus if they have a weight they know a lot of top prospects are in I would guess they would consider that for the next show. My hope is one of the next one will highlight their really lightweight divisions since they just added the new weights and got the addition of the WEC talent to their line-up.