Automatic Power On After Power Fail
2012 Mac Pro Reboot after power fail
Build a USB charger (AC to 5V) powered Arduino (e.g. Itsy Bitsy 32u4 3V) that looks for 5V on the Mac Pro USB port and momentary grounds the power switch.
Wire A5 to 10K resistors on USB5V and Gnd. This divides the USB 5V by 2 which will be read by the ADC.
Protect the resistors and the reset push button switch with some hot glue. Add a pin at D11.
Power from the USB charger. This powers the ItsyBitsy.
The pin on the right of the connector for the power switch has about a 10K pullup. When the power switch is pushed, this is pulled to ground.
Add a 28 AWG wire to the switch connector with a 2nd layer of heatshrink insulation where it routes out the front panel.