I can probably agree with Indirik's statement that it would overcomplicate game mechanics and best for in-game enforcement, but your claim of honourable oath breaking is a different one all together. In a serious medieval setting, it can never be 'the honourable thing to do'. The reason why you broke your oath doesn't matter, such a question is irrelevant, as oath breaking is not just breaking a social taboo, a mere promise of fealty, committing perjury, or treason against your liege. An oathbreaker in medieval times was traitor to the very Gods themselves (guaranteed a seat next to Judas and Satan), causing great dishonour and consequently denoting you as the lowest of the low; an accursed, even filthy peasants could kill you with impunity.
Unless the party changing allegiance felt that their liege had broken their end of the oath. I'll also point to the countless examples of vassals rebelling, etc throughout the middle ages who weren't viewed as Judas -esque (unless they lost).
