Are you part of his faction? like as a mercenary or another vassal? --> then that should override and let you enter.
(so you can use that as a workaround perhaps, just join as a mercenary, then leave if you want to after).
The other workaround is to improve relations with the lord with other party members of his clan.
Like if he have another party member in a town you can enter, talk to them - barter 5000-10000 denars should usually get you +2 to +5 relations.
Females offer the quest "Lady's knight" that gives like +7 relations to the clan(I think).
Theese are workarounds, I think that the no-entry to the caste "work as intended" but in this case it do break the "game" for you.
I thougth in general that you didnt get offered quests by lords or ladys if they disliked you? I have gotten "refused" cause they dont trust you or dislike you(note I had alot higher - with them, but was offering to fix the issue with haveing negative relations with the clan by doing a quest)
(I gave them 10k and the matter was settled heh)