While adjusting to the security mask on real, I noticed that mplayer couldn't depend on media-libs/realcodecs. Patch for ebuild to follow.
Created attachment 174033 [details, diff] mplayer-use-realcodecs.patch
It can't be an either or situation, since it relies on binary codecs from both packages. However, we don't need to depend on realplayer anymore, just realcodecs.
Changed realplayer to realcodecs in mplayer-1.0_rc2_p28058-r1