Is your problem solved? Beryl has compatibility problems with VLC player too. Full screen mode is blank. The bug has been reported. They are working on this. Soon we may see compatibility issue solved.
In VLC player, this problem can be solved to certain extent by using x11 video mode output. But the video in this case will be pixelated.
For Mplayer, you may try out "Unredirect full screen windows" option in Beryl Settings Manager. Give a try.
Here is the ticket of the bug:
Ticket #1874 (new Bug): Fullscreen problems with many apps