I've walked away from this idea banking on my 10lb halon extinguisher if I ever am in a position where I need fire suppression—also placed a much heavier emphasis on preventative maintenance and proper auxiliary electrical design / fab.
Most vehicle fires are caused by shoddy auxiliary electrical design & installations, things like; undersized/no fusing, poorly thought out wire runs, minimal/no protective looming, and no way to quickly and easily de-energize circuits. I've seen all of these mistakes on clients' rigs, and you know what?! people with minimal to no knowledge still install these dangerous circuits every day and don't worry about it because it works perfectly until it doesn't. Well the luck dice only roll in our favor ever so often, when it doesn't and something happens to a circuit with a large enough wire gauge causing it to short out/arc in the worst spot on your rig a devastating fire can be the result.
Because of this I planned my aux harness's build over a long period; mapping out runs, Connector points, circuit loads, wire sizes, wire colors, circuit fusing & fuse block protection before I pulled a single wire on my peg board. Just like Toyota did, I run a fusible link to a repinned factory 80 series fuse box that distributes power to all of my accessories through properly sized wires loomed together in fiberglass sheathing at heat-sensitive areas or braided PET. I tried hard to copy how Mr. T did his work at the factory it took forever but the product is safe, reliable, and serviceable.
The moral of the story is that by mitigating the possibility of electrical fires in conjunction with maintaining the evap system (primarily the charcoal canister, which prevents off-gassing of uncaptured fuel vapors) I don't lose sleep thinking about my rig catching fire anymore.
A defibrillator would be nice, but they need to be serviced/replaced like every 5 years (+/- depending on model) and really you should be checking it weekly to verify it's functional.
