Secondary power button

I have installed MAME on a BBB and would now like to us it in an arcade cabinet. To keep the BBB hidden away in the cabinet I will need to wire a secondary power button. I have found tutorials on adding a button using the IO pins, but none that would work as a full replacement of the onboard power button. I need the ability to both power on and off the BBB from this button.

Did you look at the schematic or SRM? The power button connects to the expansion headers on P9 pin 9. The signal name is PWR_BUT.