The areas mentioned are indeed protective coatings for exhaust areas. The vertical stab is for the empennage anti-icing heater, which burned fuel from the cross-wing manifold. The left forward fuselage is the exhaust for the cabin heaters, which both exhausted through the same pipe on the left side. The APU/APP was in the forward lower lobe in all but the EC-97. It did not need the protective coatings because it could not be operated in flight. The EC-97 had the APU in the aft lower lobe to make space for a side aimed camera in the forward area. The wing leading edges were heated via fuel heaters in the outboard engine nacelles, which also had protective coatings.
--Dan