Mplayer has the real codec use flag. When active it requires RealPlayer to be emerged. On the other hand I have had always success with using win32codecs with real use flag (therefore providing realplayer codecs). The necessary change in mplayer ebuild is to add some detection whether win32codecs package with real use flag is installed and change dependencies and REALLIBDIR (to "/usr/lib/real"). Thank you for considering this.
Well, this can't be done until dependencies are able to check for use flags (see 2272); as you've noted, it required win32codecs to be emerged w/ USE=real; there's no way to state such dependency now. Feel free to reopen once the above bug is solved.
To clarify bit more, I meant Bug 2272
