I have the same issue.
Got a button without a LED, connected on GPIO 11 & 9 and placed this code in the boot.ini:
### in case of GPIOX.3 (Pin 11) of 2×20 pins connector
setenv gpiopower “479”
setenv bootargs ${bootargs} gpiopower=${gpiopower}
Also did the ''mount -o remount, rw /boot" before editing the file because the boot partition is read-only by default.
Otherwise, it’s not possible to save the changes.
After the edit, type “mount -o remount, r /boot” to remount the partition in read-only mode.
According to the information found on the internet, a press of two seconds should activate the Odroid, but mine is not reacting at all. This is probably because the GPIO is not activated/enabled in Batocera.
I would love to have the GPIO functionality: some (KKSB) cases have the power button built-in and that should work imo.