Just an idea about something that was used in the early days of automobiles: engine exhaust. It could be routed into the cargo cabin in case of suspected fire. It's oxygen is already depleted, and the volume needed would cool quickly enough.
Oh, and it would be a lot cheaper than nitrogen for tank ullage inerting.
Before you get all wound up, think about it, seriously. I pressurize the fuel tank in my Model T Ford to 2 psi with exhaust gas. It feeds the carb, and provides inerted ullage (space above the fuel.)
GB