I've installed media-fonts/dejavu and then I couldn't select those fonts in kcontrol. Solution was run fc-cache -f as user under which i run KDE (maybe running fc-cache -f is not right way). So i think put einfo about it into fonts.eclass postinst would be fine (automatize it may be problem, becuase it has to be executed as user, but i'm not sure about that).
You should not have to run fc-cache at any time, even if it is not in the cache a new font should get picked up in a new application instance. For older versions of fontconfig it can actually be somewhat harmful to run fc-cache on gentoo.
