IIRC, I read that mounted combat is available in Lord of the Rings, with the opinion that it was not well done. I can imagine the problems with melee, but perhaps ranged would be not too difficult. Limit it to short bow and at full halt, maybe at alacrity - x? Keep the animation short and simple; arm and bow out, arm back to mount.
I think it would add a lot of spark to the game.
Coppercat