Is this an issue for player character marriage, your clan member marriage or both? As player character marriage negotiations are normally a direct interaction with the intended followed by clan leader negotiation, but for your clan members you make all negotiations via the targets clan leader.
I have unsuccessfully experimented with attempted rescues of the intended marriage target and clan leader of the intended, but those dungeon guards fight better than the same troops in the arena or on the battlefield. Must be bigger incentive. Which means waiting for their escape (or clan funding release).