[folivier]

I think the biggest problem with an engine compartment fire is the amount of air moving through the compartment. As Steve mentioned above. Halon causes a chemical reaction to bind up the oxygen so the fire stops. That's what makes it so dangerous to us humans. CO2 is heavier than air and blankets a fire thus cutting off oxygen. Dry chemical may be the most effective but does make a mess that is hard to clean.
But again the problem is if driving the amount of air moving through the engine compartment would tend to wash out whatever you're using to put out the fire. Driving with your rear camera on may give you an early indication of a fire.
Maybe adding a temperature sensor would help? If the alarm goes off you got a problem!