@flame922 You may be able to achieve that using the script mechanism: see Scripts on EmulationStation events in the wiki.
The sleep
event is triggered when the screensaver kicks in, so how about creating a script named e.g. /recalbox/share/userscripts/playvideo[sleep].sh
which launches your video.
On RasPi you can play videos with:
/usr/bin/ffmpeg -c:v rawvideo -pix_fmt rgb565le -f fbdev /dev/fb0 -re -i $file
or on PC:
/usr/bin/mpv --quiet --alpha=blend --loop $file