There is now byobu-5.27, but the Gentoo ebuilds are still at 5.18.
I successfully tried to copy byobu-5.18.ebuild to byobu-5.27.ebuild in a local overlay and I'm now running byobu-5.27.
So providing more recent ebuilds should be as easy as copying the existing ones.
I'm not exactly sure about the right dependencies. The Byobu homepage doesn't say anything about specific screen or tmux versions. However /usr/bin/byobu-ctrl-a of byobu-5.27 does require at least tmux-1.5 (no problem for us, as tmux-1.6 is the oldest version in Portage anyway).
+*byobu-5.27 (15 Jan 2013)
+ 15 Jan 2013; Justin Lecher <firstname.lastname@example.org> +byobu-5.27.ebuild,
+ Version Bump, #452140; move to python-single-r1.eclass