Well you're a scripting buff, Chainy. Don't you think you could write a short simple script that could implement a new animation if levitation spells is cast? I mean, can the engine check for what types of spells have been cast? If it can, then would not it be easy to script if levitation is cast, to enable new walking animation?
I don't think there is any way to change animations in-game. Animations are pretty limited as far as I know, and I don't think that you can even have more than one for any given action. I don't know of a way to disable them either. The engine can very easily detect what spells or effects have been cast though.
Unless you play in third-person, or if you can see your legs in first, it doesn't have much gameplay effect anyway. Still, I wish it were possible.