A global login is a good idea and would REALLY help
For example, there is another GNU GPL game out there c alled BZFlag. At first it was so tiny (roughly the same stage PB is in right now) that cheating/impersonating was no big problem.
About 6 months ago, the game started getting REALLY popular for online play. Cheaters inevitably followed. So, the bright developers came up with a global login system. It uses a PHP file and the forum user database to log you in. You use your username and password you use to log in at the forum and it authenticates you. Since it is unlikely for anyone else to know your password, it is semi-foolproof
I think it is also tied into the forum group system, so PRESTO dynamic groups/permessions...
It would probably take a good deal of work...BUT the list server is open source, and released on the sourceforge CVS. Look in the project BZflag and the cvs module 'db'
Just my 2 cents...