In the user scenario it would be enough to take current epoch time + a random number combined. Or...why not just force players that are not logged in to type a nick for that given game? I will be used while they play and disbanded when they exit. Or better yet: Let the script pick a name for them randomly from a pre-written list - that way at least all would have nice looking nicks even when being guests. Are all nicks taken? Add a random number or whatever, or better yet: Don't allow them to play - make 'em register
A limited pool of guest accounts sounds nice...why have the server encumbered with a zillion "guests", what would they contribute and how would they get a better understanding of the game than, as asp wrote, them watching a nice demo-game in HD? = P