Mutually Exclusive Public Usergroups

[the Sandman]

I'd sure like to see a hack which gives the option to limit membership in Public Usergroups so members can only be in one at any given time. Ideally, this would be done in sets such that within the set, membership is mutually exclusive (can only be in one of the set at a time), but membership would still be open within another Public Usergroup (single or set).

Example - Say I want my registered members to be able to join only one of 3 Public Usergroups with different colored usernames. This would form a set, and within the set they could only sign up for one of the groups. Say I also want them to have their choice of 3 different Private Usergroups with separate private Forums - this would form another set and they could choose one of these as well.

This would make the Public Usergroup system much more powerful and useful IMO...