Latching switch to single pulse circuit? (monostable multivibrator with 555?)
Since the power-down signal comes through a GPIO pin, your code can interpret either a high or low as the power down command.  With the right code, a GPIO low means power down.  Now the center (pole) of the switch can be tied to GND, both throws are pulled high with resistors, waiting to be pulled low, and you now have the correct signal polarity to trigger a 555.