I hope everyone here understands that positively identifying a player %100 of the time is just not possible.
However, we can narrow down the avenues of attack. Take the players GUID, in this instance we'll say its XXXXXXXXXXXXXXX
Then, take the first 3 octets of the IP the player is communicating with the server with, in this instance we'll say it's 207.68.173
Mash them together:
XXXXXXXXXXXXXXX20768173
The result, XXXXXXXXXXXXXXX20768173 is used to identify the player. Hell, have a var that allows the operator to use class B subnets in case its needed:
XXXXXXXXXXXXXXX20768XXX
The result is a number that is partially generated by the client, and partially generated by the network infrastructure(the IP). With the client creating a hash based on the servers IP, no 2 servers have a unique GUID for the client. With the server appending the source IP to the GUID, even a hacked GUID would only be useful from a class C subnet (254 usable IPs) or a class B subnet (62000 and something IPs). Obviously this isn't really true in that 13.23.76 and 132.37.6 would appear the same to the server, maybe replace the . with a lower case c to delineate the octets.
GUID brute force attacks would have considerably less chance of succeeding and the server can be more sure that a player/admin actually is who they say they are.
If you still think I'm sane, follow me while I wander to where fixing an issue this would create (for some) might lead, aside from the mountain off issues I'm sure I'm missing or glossing over

A few ISPs use Class A subnets for their clients, but I don't come across this very often at all. Still, some admins would find their rights getting screwed up at times.
So, the admin goes and logs onto the forum for the server while connected to the trem server and clicks a button. The forum software has authenticated the user and knows their IP. Using pyquake and python, (only scripting lib for quake I know of right now) the forum server determines which slot on the server the admin is using by comparing the IP connected to the forum vs the IP connected to the server and grants that slot the Admins pre-assigned admin level and presto, the Operator has successfully avoided that nasty work thing!