Perhaps a repeater system with a flashing emergency light topside? It would be more expensive to outfit.
As to CO2 / CO detectors, I don't think they're necessary. You do need to consider gas-safe reentry procedures if there is a fire. How long will it take for ventilation method X to change out the air, how will you sample for gases, etc. On modern boats, they use hand held units for sampling. Does anyone know a civilian source?
I am working all this into a document that I'll publish in a few weeks. So far I'm at 10 pages with quite a bit left to fill in.
Brian