I was off the understanding that the FAA required full face mask above 18k and I did actually have one of your favorite references for this IO but just not on my laptop! The CAA have made no recommendations other than the requirement to use O2.
However you are right, there is no reason why a cannula won't work well above this level, it depends on the individual. I have no idea if they will work at 25k, I have not been that high un-pressurised but await the results of your self experiments with eager anticipation!
But the rules are supposed to be aimed at the mean average....