I do agree. I thought about it because it springed up here, and I think it is bound to happen at least a few times, as nothing can stay totally secret.
This could make a lot of trouble to a covenant, and could be both a great "ennemy" or "favors" flaw, or secret (the mundane knows this because of a character)
I think that this would be the most well known bit of the code outside of the order. Magi would routinely use it to turn down requests for assistnace from others. "I would help my lord by my oath forbids it on pain of death."
A dangerous gamble - as it might not only earn you the blackmailed as either accepting grudingly (while plotting your demise), or flat out denying while moving on to razing your home, burn you crops and kill your livestock (untill "corrected" by the Quaesitors). And finally you might risk the entire Order, in the form of the local Tribunal, declaring you a capital 'E' Enemy...
But it makes for a wonderful plot all the same... 
I welcome you warmly amongst the rank of the Regnants, Sovereign Tyrrell
Thank you very much.