I definitly LOVE the idea of making more variety within the horse catagory, particularly the higher-tier horses. This is MOUNT AND BLADE after all.
I think draped horses would be awesome, and any western knight characters will pick them up in a heart-beat. You said heraldic drapes would be too hard, which I find sad, because that would be simply orgasmic. Also, would these draped horses just be like another version of the Destrier, or would it have more HP/Armor--a result of perhaps extra armor. It may fit snugly inbetween the destrier and warhorse stat-wise.
The draped horse could use a bit more wear-and tear on its drapes. Perhaps muddy up and scratch the bottom, and add detail to the drapes and horse in general. Perhaps more straps and harnasses (the seat seems to be just sitting on the cloth, unsecured), and jingling bells (like the destrier on the M&B loading screen). Perhaps a tiny bit of head armor, or ribbons. It's your model, at the end of the day, I suggest having fun with it and making it look pretty
Even if the horses were simply different versions of what we already have, I whole-heartedly support ur endeaver Mr. OP. I luv horses.