Do thermal cutoffs exist that would work with a line that is 40-75V 0.5A and a max temp of ~85°C?
If it is a signal or LED driver it is still AC or DC based current, enough for a thermal disc to pass through if contacts are closed. Voltage is not an issue here as 70 volts is not that much at all, as the cutoff will likely have a 250 VAC rating to cover most any common use.

You want one with N.C. contacts that open when too hot. You should be able to find one close to the cutoff point you want. Once OFF they have to cool by about 5 deg F to reset.

This was the best list I was able to find. It seems they are not as common as in the past. I suspect adjustable electronic types have replaced them. As it is mostly what I find and becoming more popular are solid-state resettable polyfuses. Not designed for high voltage or high current they can drive an AC or DC relay that has the right voltage / current ratings. An exception would be in the link below which leads to high current 250 volt PTC's that reset when cooled off.

Link to a better solution