Resolving betrothals 'stuck' due opinion change

Well, from my own experience it's only matter of time before AI liege will suggest marriage himself, even if current opinion and succession line situation indicated it is impossible. It happened to me multiple times - most notable situation playing as a Duke of Greater Poland, having my daugther married matrilineally to HRE heir, becoming empress in the same year.

EDIT: Actually I had situation when bethrotal got locked due to husband-to-be becoming knight of military order. I've just broken the bethrotal, it's better to lose some opinion and try another bethrotal instead of losing benefit of marriage on one of your family members.