I've checked out the gnome-overlay and get a segfault while opening a message with some kind of encoding or attachment. Did you guys realize the segfault bug https://bugzilla.gnome.org/show_bug.cgi?id=612082 from the evolution bugzille site. This patch fixes that problem for me. May you want to add that patch to the current evolution-2.30.0.1 ebuild.
Thanks for the heads up, I have pushed revbumped evolution in overlay.