I'm thinking Wilmer Valderrama as Kahn Noonian Singh during the two Eugenics Wars books, or at least the second one where he's older.
I dunno, I can't see him as anything but Fez. I would just keep expecting Hyde and the rest to walk in and catch him eating sweets while perving over Jackie or Donna ...
'Eet ees very cold in thpathe! I weel aventhe mythelf on Kirk. But first I weel eet theeth doughnut'