GD&L,
Thanks for that link. I looked into it further but it doesn't support my suggestion. One of the modifications mentioned which allows the A319's altitude of 41k is to do with the placement of the fuel probes - I suspect the other mod is similar.
If a different oxygen regulator is fitted to the EROS mask it should allow operation up to 45k or beyond (provided the airframe and engines do not then become altitude limiting at lower levels).
I must get out more.