having flown a lot above FL450, we had a device ( actually two) which were concentrating the O2 from the cabin, and through an humidificator to nose canula.
These little tubes were not interfering with normal ops, not picking into the O2 reserves of the plane, had no chemical in it. However when a pilot was leaving his seat, the other one was putting on the real thing.
even this a relatively low cabin altitude, this extra O2 was really nice on 12+ hours trip.
Cannot get my hands on the documentation of it but was surely STCd, since it was hard installed in the plane. GLEX by the way.