Sadly that's not the one I was looking for. From what little I remember of the fic it started off with Harry in Ankh Morpork all grown up and being a bad ass. I don't think Rincewind was mentioned at all.
The search option at fanfiction.net didn't seem to turn up anything like what I remember. I think it was part of some author's short story or drabble collection. This is going to drive me nuts :headbanger: