> Wood will not self-combust until reaching around 300°c

Staying below the "fail fast" temperature does not make it safe: https://www.youtube.com/watch?v=FzF1KySHmUA

The question is, how will it handle years of the daily heating and cooling cycles that the cooling system will undergo? I don't know and likely no-one does ... because it hasn't been rated or approved for that task. it isn't even a standardised part, how can it be?

>The question is, how will it handle years of the daily heating and cooling cycles that the cooling system will undergo?

...you'll find out at the regular inspection and service that you are required by law to have at intervals far more frequent then "years"? That's part of the point of them, both to notice fleet issues that will be reported back up and conversely to implement directives ("it needs to be replaced with this updated strain relief at the next service") coming down.

This is only visible by substantially disassembling the car (the person who originally reported it found it when fixing panel work). As far as I know that's not routinely done _anywhere_.

A great many places, including the majority of states in the US, have zero requirement for vehicle safety inspections.

Even the states that do require inspections are inspecting things like brakes, mirrors, and signal lights, not disassembling vehicles. Here's an example of what Massachusets does: https://www.mavehiclecheck.com/motorists-basicinfo/