There is a program Mii Tranfer for Mac which can get your Mii characters from remote. Then you can edit using an online Mii editor, Mii Editor to further edit your face. I think it would be fun to reuse your character in elsewhere.
(2007-03-16) Mike has fixed PPC version problem. You can download 0.1a from here. Remember to read the instruction before use! 🙂