I have not yet found a reference for cannulas being not approved above 18k feet (obviously this is FAA - the CAA never got this far into general aviation) but it is a widely quoted figure.
But they certainly work. I was at 95% saturation, 75 heart rate, at FL190 the other day, using straight cannulas with demand regs, for an hour or two. The bottom line is the blood saturation - if you are getting above 90% than whatever you are doing is obviously OK. If you could get 95% by sticking the cannula up your back orifice, that would be fine too
A TB21 has an official ceiling of 25k feet and I am certain cannulas would work there just fine. One will be using a LOT of oxygen up there, but a mask will use a LOT of oxygen anyway, a) because it works inefficiently and b) because one can't use it (supposedly) with a demand regulator.
EDDNR - there was some discussion of this on the socata.org users group. You aren't the only one stuck with the fitted system. There may be other bottle options, perhaps even with a kevlar bottle which I believe was fitted to later TB21s. But to be honest most fitted-oxygen owners find refilling so difficult around Europe that they use portable kits anyway
