either via a script, or better yet, a built-in dialog similar to the Address Book's Colors tab. An example of its usage might be to append a nick's idle time in seconds/minutes/whathaveyou.

ex:
@onick1 (34m)
@onick2 (25m)
@onick3 (3h23m)
+vnick1 (15m)
lamernick (3m)

you get the idea. perhaps it could allow you to set color codes or bold or underline or whatever to nicks that meet a certain criteria, like nicks on a channel that are idle for over an hour are displayed dark blue.

the only thing I can think of that might make this a security problem is if a script that someone else gave you tries to display a completely different name where a person's real nick should be. it should not be difficult to prevent such a thing from happening while still giving us great flexibility in the presentation of the nicklist.