Positive pressure ba sets primary purpose, at least in fire fighting kit I have used, was to flood the mask with O2 in the event the face seal was compromised, ie. visor smashed. Your talking comparatively sort time scales, gas in motion, and therefore little opportunity for complex gas interaction. :huh:
I suspect you are spot on FM,
This article also says it well, particulaly the very last sentance!

So even though there is CO2 underpressure it will not stop O2 dissolving into the beer.
I've had to do a bit on gas laws for my scuba diving.
Ian