Of course it's a speculation - I'm not a time traveller
I'm just suggesting that making separate "mounted" sprites for actors will cause too much trouble while this option of a fight cannot be used in caves or buildings. Changing the spriteset and few attributes (like movement speed) can be easily done just with eventing/scripting.
More than possible compatibility issues with Visual Equipment, although I don't know that for sure (I'm not that familiar with the script).
If you want it, I'm not stopping you, it would be an awesome feature, perhaps - but I don't think there's any real demand on it.