I wouldn't be worried about overheating - most hardware will shut down for safety way before you reach temperatures capable of igniting a fire. Instead I'm worried about catastrophic failure (especially of power-related components such as PSUs or that UPS) where the thing explodes out of the blue and spits out sparks and flames.
In this case, you'll very quickly get a self-sustaining fire (thanks to all that firewood around it) and will need to actively extinguish it - turning the power off or starting the AC will not save you.
The best solution, short of an active fire suppression system such as sprinklers (which obviously come with their own problems) is to put the hardware in a place where you can have a device catch fire and be confident that the fire won't spread or get out of hand.
In this case, you'll very quickly get a self-sustaining fire (thanks to all that firewood around it) and will need to actively extinguish it - turning the power off or starting the AC will not save you.
The best solution, short of an active fire suppression system such as sprinklers (which obviously come with their own problems) is to put the hardware in a place where you can have a device catch fire and be confident that the fire won't spread or get out of hand.