It would, though, have to be a most exceptional situation. Suppose that a prominent public figure found himself in a position to break his promise so outrageously that several others would decide to keep their promises in future. But such moral conversions are so unusual that it would be unreasonable for him to elevate his act in this one case to a policy for his moral life. If I, situated as I am, were to break a promise in order to bring about more cases of autonomous promise-keeping, then, unlike the public figure, I should merely be dropping my act into a causal stream in which so many other eddies are at work that I could have, at best, only the faintest hope of producing the desired effect. Ought' implies 'can'.
All moral theories work with a picture of what lies within human capacity.
But acts like maximizing the universal observance of promise-keeping are not in our repertoire. I can aim at doing something when my chances are less than certain. But if my chances are only one in several million, then I cannot be said to choose to, or even to try to, do the act. There is something unreal in consequentialists' adopting as one of their goals in life 'Promote promise-keeping impartially'. It is not the sort of action-guiding goal that one would ever give to, or adopt as, an agent.