This would be possible if you check presence of OMXPlayer process. It's not an elegant method but it might work. But keep in mind you will loose opportunity to play video snapshots in gamelist
So the better way would be an implementation into the C++ code of ES itslef.
PSEUDOCODE
#!/bin/bash
# Run this script with root user or sudo command
# Initiate GPIO x ouput
# Exports pin to userspace
echo x > /sys/class/gpio/export
# Sets pin x as an output
echo out > /sys/class/gpio/gpiox/direction
while true
do
# Slow down the whole thing a bit
sleep 5
# Suggest running oxmplayer instance
omxpid=$(pgrep omxplayer)
# Set GPIO x high > switch off screen
[[ -n $omxpid ]] && echo "1" > /sys/class/gpio/gpiox/value
# Set GPIO x low > switch on screen
[[ -z $omxpid ]] && echo "0" > /sys/class/gpio/gpiox/value
done
