Most challengers will be available for battle at meetups or random leader sightings I'd imagine.
However, it sounds like you want to easily recognize a fellow challenger if you're just roaming PAX. I don't know if unique id numbers would be the best way. If you get stickers made where will people wear them?
Also there's probably going to be a good 75-300 challengers I'd assume throughout PAX. And by the time PAX starts probably only 10 tops will be on these forums.
Not trying to discourage you but maybe have a simple system set in place. Like have leaders hand out whatever you want challengers to wear as they are challenged instead of you trying to randomly hand them out.
I don't know why you want unique id numbers but maybe have people wear a big pin on a specific part of their body. Like right shoulder or something.
Also here is the badge icon that is being used for the Trading Cards. Would probably be best to use the same design.
[img3x0]http://paxpokemonleague.com/images/PokePatchPokeBall.png[/img3x0]
And here is the link for the PAX Trading Cards in case you wanted to learn about them and possibly order some.
http://forums.penny-arcade.com/showthread.php?t=129849