I just remembered something!
Last session, while they were looting the crypt, the PCs woke up Sasha Ivliskova (Strahd's neglected bride). With a few good Persuasion rolls, they talked her out of attacking them by distracting her with the news that Strahd was marrying someone else, and sent her upstairs. I don't think they had any thought beyond "Maximum chaos LOL" (with a side order of "Don't wanna fight her"), but maybe I can use that. Like, the ceremony gets put on hold while all the guests comb the castle looking for the intruders. Guests include werewolves, Barovian witches, and other dangerous folks.