I was looking online a while back and i thought i remember seeing something like a way to wire a button that when pressed runs a shutdown script. i have tha reset switch figured out that was simple. im also looking for how to wire an LED power indicator. One i get this figured out my build will be complete.
SNESDev (https://github.com/petrockblog/SNESDev-RPi) implements a virtual keyboard that executes the shutdown command when pressing the button of the RetroPie GPIO Adapter five times. Maybe you want to have a look at the sources of SNESDev.