Mplayer supports, and has for a while, an audio filter that allows the use of LADSPA plugins, and there are currently no options to build it. Following ebuild (based on mplayer-1.0_rc2.ebuild) adds an "--enable-ladspa" string to myconf, and a dependency on ladspa-sdk, both dependent on the "ladspa" use flag.
Created attachment 134874 [details] mplayer-1.0_rc2 with optional ladspa support
Fixed in cvs, thanks