CO detectors are life safety devices. Shouldn't this circuit be incorporated into the building's fire alarm? As a standalone system, using N. O. contacts as stated in the original post: the devices, wiring, and the presence of control power (whether AC or DC) will not be supervised. Connected to a fire alarm input module or zone and using an end of line power supervision relay, or using addressable CO detectors all of the above is supervised. The standby battery 24 volt DC power from a fire alarm ensures occupants are warned of CO during power failures when the exhaust fan will be down if there is no emergency generator present.
But hey, as Dennis Miller would say......maybe I'm wrong.