Cavalry WAS actually used on sieges - but as support.
1) By attackers - to charge from opened gate, make a raid and return. Can be shown as attackers going out and charge as they do now, but if only maps are made more open on sieges.
2) By defenders - to deliver messages and for reconnaissance. Hard to implement in cRPG, as there are global chat, TS and Spectator.
3) Also by defenders - to cover flanks and rear to avoid attacks from armies outside castle. Also impossibru.
So, the only way to make cavalry more useful is to make more open spaces around castles - and I am completely for it, even though I rarely play cav even when I have money to sacrifice and good moment.