Why does that AI make the easy so hard? 
You already have the 5V and the 12V leds in the MacArthur HAT. If these leds are hidden by a casing or some panels, you can add a LED + resistor to the 3.3V/GND pins. When the shutdown sequence is complete, it will turn off. You can also use 5V/GND pins or even the 1W connector, see 1W alternative uses in the manual.
EDITED: Actually the Raspberry red LED will also do this job. when the shutdown sequence is complete it will turn off. You should not see any LED on in the Raspberry or the MacArthur HAT.
You already have the 5V and the 12V leds in the MacArthur HAT. If these leds are hidden by a casing or some panels, you can add a LED + resistor to the 3.3V/GND pins. When the shutdown sequence is complete, it will turn off. You can also use 5V/GND pins or even the 1W connector, see 1W alternative uses in the manual.
EDITED: Actually the Raspberry red LED will also do this job. when the shutdown sequence is complete it will turn off. You should not see any LED on in the Raspberry or the MacArthur HAT.