In all my years playing games - Baldur's gate, Icewind, Oblivion, Morrowind, Skyrim, Fallout, Mass Effect, Assasins Creed, KOTR there has only been one that I can remember that at advanced levels allowed you the ability to fly: Might and Magic VII I believe.
I understand it's difficult to get it to all work - it's worth it! The ability to fly around to places that you normally couldn't get to on foot is WAY COOL. I'm always hopeful, and alwyas disappointed, whenever I crack open that new game and it isn't there. :|