Would be easier just using a single relay, a D-Type flipflop with a Set-Reset input, and a transistor/relay. That way the power key can trigger the flip flop, latch it in and engage the relay. Its always on no matter what. Then the VIA pin is hooked to Reset, so when mac shuts down, it clears the flipflop which shuts off the relay. Relay then cuts power to the mac, allowing the Reset line to go back to a floating state for you to press the power key again. 

Now, the other option is use a microcontroller, that way, you can press and continuously hold the power key to force a power-off in case of a hard freeze.