There may be an easier way..
Use a relay in series with a diode and a resistor, and with a fat electrolytic across the coil.
In fact, I suspect something like a triac would work in that mode, too.
But I still like the thermistor the best.
Less components, and possible to use a really big fat one!
The CL-80 from here looks about right
I think I would be tempted to put it in the actual backing box of the switch itself.
One end into the switched live terminal, and the other crimped to the switche live wire. Then glue some insulation to any nearby part of the backing box in case it touches that.
Or, if it was not appropiate to use the backing box, use another backing box with a 4 terminal block in it, and mount that where it can get hot in peace!