Spent hours and learned the hard way that you can't add it to the retropie/gamelist.xml anymore in 2.7.5
But you can add it to any other system gamelist.xml that has roms
Really needs a default folder as an option but for now just add the tag to any of your system gamelists
I don't use videos in ES because of the extreme heat
But I link 5 videos without names/paths to be discovered by the random video screensaver by simply adding it to the bottom of the nes/gamelist.xml file
<gameList>
<game><video>/home/pi/RetroPie/splashscreens/ss1.mp4</video></game>
<game><video>/home/pi/RetroPie/splashscreens/ss2.mp4</video></game>
<game><video>/home/pi/RetroPie/splashscreens/ss3.mp4</video></game>
<game><video>/home/pi/RetroPie/splashscreens/ss4.mp4</video></game>
<game><video>/home/pi/RetroPie/splashscreens/ss5.mp4</video></game>
....
</gameList>
Works perfect every time and oddly don't overheat the pi while playing.
