I mean. It's siege... The more useless cav there are in your base the fewer there are in the castle. I think it's a wash in siege because the stupid cav players leave all the gates open. Which helps disproportionately to the amount of kills they get hurts in a game mode with infinite lives. It's let me capture quite a few castles that otherwise might have held when attacking, but usually leaves me on door duty when defending. If they don't end up changing things about how cav are outfitted (and they could still loot anyway), the solution that comes to mind is on the mapmakers. If you look at the WB native siege maps and the BL siege maps (so far) there is a lot more stuff in the way to break up cav and provide cover. It's not perfect and probably wont ever be perfect, but they provide something against cav leaving the castle. Best bet for attacker is to stick to cover, use the ~ key and pay attention, and keep advancing; cav are looking for people who aren't looking.
Thinking on it more as I type this, maybe letting defenders sally out (spawn cav) for the first 5 minutes, or something. Then imposing either a restriction or limit on cav. But then that would be unfair the the kherghit (I got to learn the new names) faction, but then they don't belong in siege anyway; or at least at a reduced percent chance to appear. Dunno if that would work, probably not in the way intended.