I think that Mode S could definitely be exploited for this purpose. A message could easily be sent as part of regularly broadcast telemetry that would present on the controller's display that the pressurization system has been compromised. If after being queried, the crew does not confirm they are using supplementary O2, then the controller forces the flight down to FL130, or at least an altitude low enough to bring the pilots back some of their brain function. Obviously factors such as MEA, MAA, and MRA need to be considered. (Terrain)
I know that there is a culture prevalent among pilots that does not embrace the idea of transmitting data about the aircraft beyond what is currently available, but honestly it is about CDM. System Wide Information Management (SWIM) or Cloud, is being embraced by ANSP's, airlines, and others as the way of the future. The more data a controller has access to about a flight, the better the SA. My two cents worth.