It's been my experience that once you've been unhorsed you're pretty much screwed, as the infantry you were riding down mobs in to bludgeon you to death with a vengeance (I've been both the bludgeoner and the bludgeonee, so I've seen both sides of this phenomenon)
That said, I don't think impact damage from being unhorsed is going to bring anything to the dynamic here. The only thing it would add is allowing a lone infantryman to have an easier time killing a cavalryman on the off chance he manages to unhorse him. Rather, what I think is needed is easier ways for a group of infantry to knock a rider from his horse, or kill the horse outright, thus allowing the beatdown to commence (though it might be a moot point once the hitbox issue is fixed)
One of the suggestions that's come up repeatedly is setting your spear against a charge, but that's been discussed to death so I'll only mention it in passing. Riding into things like a low hanging branch, or the ruins SteveO mentioned should also knock you clean from your horse. Basically if your hitbox impacts some scenery that your horse doesn't, or something along those lines. Though if that's the case, you might need to introduce a way of crouching down on your horse, and suddenly this gets way more complicated than I meant it to be.