Rhythmbox needs dev-python/louie and python module "coherence" which is not even in the portage! Since I couldn't go any further testing, there might be other missing deps as well. May be a use flag for this plugin as well?
Not only a use flag, it just needs dependencies to be in tree.
+ 03 Mar 2010; Gilles Dartiguelongue <eva@gentoo.org> + rhythmbox-0.12.7.ebuild, metadata.xml: + Add coherence support, bug #293051. Dependencies are now in tree, added support to rhythmbox.