Summary pretty much says it all. ffmpeg ebuild doesn't seem to provide any 'ffmpeg-config' binary. Coriander's configure disables encoding to mpeg formats as it doesn't find ffmpeg-config. Furthermore, maybe coriander needs to get a patch so ffmpeg is enabled/disabled with a configure flag, but I'm not sure about that.
Yep, we need a configure flag, and the configure script should use PKG_CHECK_MODULES (i.e. pkgconfig) to check for ffmpeg and fall back on ffmpeg-config as this is the old way. Maybe you could try writing a patch for this? :)
Fixed in coriander-2.0.0_rc1-r1