file ~/.kde/share/applnk/System/ScreenSavers/Zeroth.desktop:

[Desktop Entry]
Encoding=UTF-8
Exec=mplayer -x 1440 -y 900 $(/usr/bin/randomfile /mnt/dat/quake_vids/ .avi)
Icon=kscreensaver
Type=Application
Actions=Setup;InWindow;Root;
X-KDE-Category=OpenGL Screen Savers
X-KDE-Type=OpenGL
Name=Zeroth

[Desktop Action InWindow]
Exec=mplayer -wid %w -x 1440 -y 900 $(/usr/bin/randomfile /mnt/dat/quake_vids/ .avi)
Name=Display in Specified Window
NoDisplay=true

[Desktop Action Root]
Exec=mplayer -rootwin -x 1440 -y 900 $(/usr/bin/randomfile /mnt/dat/quake_vids/ .avi)
Name=Display in Root Window
NoDisplay=true