Been done. The Sturmey-Archer ASC-3 was a 3-speed internally geared fixed gear hub (like the much more common AW-3 but fixed). I hear they're hard to find now. Personally, while it seems like a cool trick, it doesn't appeal to me. It's still a planetary gear which means quite a bit of mechanical inefficiency (relatively speaking) and besides, a large part of the zen of fixed gear is finding the gear that you can push under all or nearly all circumstances. For the zenness, it seems like a cheat to shift.