This is something that would require a different horse for each combination of horse+armor.
Unfortunately, not for Warband, maybe in the future, unless they really want to make all those variations. (Same goes for things like wearing surcoats over any kind of armor you want, or armors where specific areas can change color via a swatch pallet, etc.)
However, I kinda assumed that many armored horses were just hunters with armor on, under the assumption that they're the only ones that could have armor that heavy. A courser could have some kind of light armor on it perhaps, maybe the other horses too. It would still require adding a whole bunch of horses to the game. It's just a question of whether or not they are willing to go through the trouble.