I'm at a loss here. I know how it could be done, but it seems like a bit much for something that can be fixed by modifying how a character is saved. You could just make a flag set on new players (like unauthed for example), that so long as the flag is on them they are not saved. Warn them about this, then if they're ready to be a part of the game, they can remove the flag.
If you're trying to make it so that the guest is a pre-made user, that's a bit different. It also may be difficult as you'd have to more than likely handle saves in a weird way. You could copy nanny's character creation though and make a 'create_guest()' function that's triggered should the player type in 'guest'. You may want to allow them to specify a name to go by, or just some how keep a log of how many guests you've had and possibly append numbers to the word 'Guest', like TinyMUSH does. Not sure if numbers are valid in names though.