mplayer has nvidia and i8x0 USE flags to provide support for XvMC. Instead, the mplayer ebuild could be updated to use the VIDEO_CARDS settings. Also, the ebuild for xine-lib used to work in this manner but was updated so it's possible that the changes to xine-lib's ebuild could be used as a guide for changing it in mplayer.
-r1 removes everything in favor of xvmcw