Oldebloke,
This is an interesting question because I was thinking along similar lines of thought to yours. Upon reflection, It occured to me that since this may not be addressed by the flight manual or QRH, then it would be open to interpretation by all who learn of the occurance after the fact. Certainly, calling for the walk-around bottle is a good idea, but a descent to a more breathable altitude may be a good idea if the situation permits. Stopping at 310 to think things through is a wonderful idea. Terrain, weather below and flying time to a suitable alternate would have to be considered before deciding what is best to do. Continuance of the flight to destination at 310 would probably be questioned if suitable alternates are overflown though. In light of increased attention to crew O2 and it's probable role in two high profile crashes in recent years, It would be a challenging position to defend . Once safety has been satified, then the repercussions may be considered. How do you think the discussion might go?
Best regards,
Westhawk