edit /opt/retropie/configs/all/runcommand-onstart.sh
nano /opt/retropie/configs/all/runcommand-onstart.sh`
paste the following into script (it checks if the system is psx, if so, play the mp4):
[[ $1 == *psx* ]] && omxplayer -o both -b --layer 10001 /path/to/your/ps1.mp4
make it executable.
chmod a+x /opt/retropie/configs/all/runcommand-onstart.sh
The video I used was found