Thanks, Guppy. Hmm, what if the cargo compartment cameras I suggested were thermal? Would that eliminate the problem of them being obscured by smoke? And I understand that it's a huge area and the amount of fire extinguisher to cover the entire area would be prohibitive - but if the fire could be located via camera, would it be possible to have a piped system of fire retardant so that you could just trigger (from the cockpit) sprinklers/jets in close proximity to the actual fire?
It probably wouldn't put the fire out, but slowing the flames/smoke and buying 5 or 10 minutes could make a huge difference.
I do think your suggestion of sitting in the plane with eyes closed figuring out and visualizing exactly where everything is seems like a great idea.
And this is probably a stupid idea (and feel free to tell me why it wouldn't work) - but if hazardous material HAS to be shipped via airplane - could some/much/all of it be shipped sealed inside a fire-resistant metal container? Perhaps even vacuum packed, as most things will not burn without exposure to oxygen.