Or Billy... or even Kara at a push. Big push. Laura-as-Dumbledore works terrifyingly well. Although my mind boggles at the thought of her with purple robes and a twinkle.

That means either Zarek or Baltar would be Snape, and Tory would be an excellent McGonagall. I can kinda see Athena as Hermione and Helo as Ron...