If you answered "mu" because it embodied some intellectual principle the master would most likely detect that and reject your response. If you intone "mu" as a genuine expression of your existence/reality at that moment, an expression that finallly has nothing to do with the meaning of the koan per se, the master might accept it and pass you on to another koan. Maybe not.
Of course the koan is a non-issue outside the context of the zen encounter. But if you accept the premises of the encounter, if you permit yourself to enter the trap, to become engaged then you must free yourself from it, but you won't so long as you expect the master to free you by accepting your response. I think he passes you when he realizes that you have already freed yourself by transcending the trap. Easier said than done, because the master has ways to keep you engaged.
I think your mu was an enlightened response, even if the master rejects it.