Good news: As of yesterday, there is an ebuild for 2.8.0 in Gentoo
portage. Previously, the latest versions were 2.4.2 and 2.5.2. It's
good not to have to roll my own ebuilds anymore.
All Gentoo Lilypond users who update their systems regularly will see
this if they have the unstable flag set for this application.
Unfortunately the latest version marked stable in Portage is still
2.0.3. I hope this changes soon.