Is there any combination of info in UT that could make someone uniquely identifiable, maybe with the combination of system information (like processor, or other system info that programs have access to)?

You could use that to generate a key, and store it in an .ini or something. (I don't know what powers you have from within UT for storing info)

If any of their info changes, just make the player re-register. If they play from multiple computers make them register from each of them.

I don't think IP's are the way to go because of dynamic addressing, but you could have an option when someone registers and they have a static IP and want to use it as part of their authentication they can. But that could get messy if people don't know what they are doing.

You could use a kerberos-type setup where the central server is a ticket authority and the ticket gives a player permission to play at one server or a "ring" of servers and if he wants to play at another server (or ring) he has to re-authenticate.

I may be taking it a little too far, but I'm not sure of the powers of uscript. just some thoughts.