Stephen, I think your idea of using a central heating valve is a good one. Obviously, the sensing bulb would need to be where the fire is. In an open boat I guess you would need to give the valve a good coat of paint to protect it but, on my central heating, the capillary tube and the bulb are both copper so they would last a good while.
On my boat, I have a 2kg hand CO2 extighisher in the cabin and two of these under the engine hood. http://www.magicmarine.co.uk/fire/Ex...herOrders.html
I do believe the CO2 is useful. If the fire is large enough I feel you are unlikely to win with any smallish hand held extinguisher. I know CO2 is not recommended for materials that one might imagine may burn on a boat but I've used CO2 on smallish fires for which they are not recommended and the dousing was pretty well instant. In a fire situation that is recoverable, I'd expect the CO2 to cool it and I'd rather not have to clear up the mess of powder. In an engine compartment, filling your engine with dry powder while it is running with surely do damage.
I do think that if you get a real petrol fire it will simply be a whoomph and your best bet would be the sea, if you are able.
My understanding of C02 and Halon replacement is that they mainly work by absorbing the heat of the fire rather than by smothering it.