If I can help in any way feel free to PM me. My understanding is that the first few (maybe as many as a dozen) MONO CAVs had final assembly done at the South African BMW plant. My car falls into that so I assume that's why I have so many BMW bits on the car, (paint, wheel bearings, switches etc.) The underside is stainless and I put undercoating down there to diminish the road noise, not corrosion protection. My guess is the factory did the same thing. It was probably unnecessry as a big difference occured when I put Dynomat Extreme throughout the cabin. You now can have a moderate conversation during spirited driving with only the purring of the 302 in the background.
For the rear deck, I have bed liner over the rear wheels with Dynomat extreme under the plenum around the carb topped with reflective heat shielding under the front of the engine clamshell and Dynomat under-hood over the collector to and around the exit vent. (See photo) Hoodliner
As for the flammability of the bed liner, some are, and some aren't so check with the manufacturer of whatever you're considering. Mine was listed as non-flammable but only when cured/dried. Before they cure they'll burn like a flashbulb though. My suggestion would be thin bedliner for the wheel wells and regular high heat primer and flat black for the remainder. Then cover that with the insulation I cited earlier.