agpukcrew, oxygen masks CAN be disconnected, however you can not shut off the oxygen supply from the oxygen generator as it is a chemical reaction.
From Vodkaholic's posting at 3rd place in this thread we have been discussing whether the masks can be disconnected, as in whether they can be detached. I said in post number 4 that they can, I also reiterated in post number 9 that they can be removed.
Here is a photo of a drop down oxygen mask:
As you can see on the right hand side of the picture, the mask has a connector which can easily be connected and disconnected from the generator. I asked an engineer today, he confirmed that the masks can be disconnected and connected from the generators.