So for a hardware monitor utilizing an Arduino, it won't increase cost significantly to go ahead and monitor GPU fan's RPM and temp of a sensor mounted to the GPU heatsink. They'd both require the same shutoff circuit.
So the parts required are:
1. Arduino of your favorite flavor - not sure on fan RPM data voltage levels, so maybe 5v input tolerance would be ideal.
2. Temp sensor:
DS18B20
3. 40x40 3-wire fan (or 50x50 would probably work and mount into at least 2 of the existing holes for the stock heatsink, though would not be recessed down into the slot).
4. Assuming the power can be cut by severing one wire on the power coming into the mobo, a MOSFET might be fine, or perhaps an opto-coupler or a relay.
5. If your Chihiro uses an ATX style power connector, you could use an ATX extension cable and hack into the wires there, to make a reversible modification... or otherwise build a wiring adapter to cut into.
6. Maybe an audible buzzer? It might be nice to hear an alarm if it decides it needs to cut power.