Could you make him take off the hat before each battle? It could be some sort of intro to the battle for him and save you the hassle of making unique character sprites.
Pretty cool idea , but not this time. Maybe for some other project. This idea is pretty neat though!
Well I don't know what you have planned for the battle system, but if its anything standard, you could probably just do the one main battle charset, and make a bunch of copies with the different hats pasted on it. It certainly wouldn't take long. Heck, I'd do it for you if you want.
Shouldn't be hard to do that myself, but well, I don't really see a real benefit to it. There is basically 30+ pose for the main protagonist (for different weapons and skill).
Also, there are plans for charset change halfway in the game so.. yeah. those edits would be wasted.