Activated masks will supply O2 until either reset at individual unit (are your cabin staff trained to do this?) or supply turned off at central point. Until then they are leaking 100% O2 into the cabin. Are you really going to go and count the deployed masks? Does that even have a point, as if deployed they are still available for use, surely? If resetting individual masks at their units, why not restow? Quite frankly, whilst theoretically possible, it all seems exactly that - theoretical. With a load of worried passengers, a diversion seems sensible.