Being very intrigued concerning 'blow-by' from the mouth while the nose is pressurized I looked up the anatomy. There's a narrowing of the passage leading from the nasal part of the pharynx down to the oral part, where the soft pallet projects backwards. The soft pallet being flexible and muscular, it can completely close and this is why it's possible for the nose to be pressurized, with no airflow, and for the mouth to be free to take in air from the room at normal pressure. Gray's anatomy shows it well:
http://en.wikipedia.org/wiki/Image:Gray994.png
The soft pallet has muscles which can be controlled to allow pressure to escape from the nose and out of the mouth at will. It's a strange sensation for anybody who never tried it because we're normally not conscious of controlling these muscles! Although I don't dispute that air can escape from the stomach and into the mouth (of course) I can't see how this can be a big contribution to what comes out under XPAP conditions.
Best,
Spotty
