Cannulas work fine at FL240 with the MH EDS system, and if you don't make a positive inhale for a few seconds the system beeps at you. Experience in a 35 h.p. single enginred aircraft, but I have to confess the engine wasn't running on that occasion above FL 35, relying on free meteorological climb for the other 21,500 ft.
I am told the finger oxymeters are temperature senstive, so if you start to get frostbite they are iffy, but wear warm gloves and they are OK. I have no first hand knowledge. Also, for some reason, US suppliers won't ship to a UK address.