mir : Mir is a display server technology ( https://launchpad.net/mir/ )
platform-api : Implementation of the Platform API for a Mir server ( https://launchpad.net/platform-api )
qtmir : Qt platform abstraction (QPA) plugin for a Mir server (desktop) ( https://launchpad.net/qtmir )
unity-system-compositor : Compositor for Mir display server that switches graphics and input between running sessions ( https://launchpad.net/unity-system-compositor )
Add an ebuild in portage :
The ebuild is now in the portage tree.
You can also use layman : emerge layman then layman -a unity-gentoo
For Paludis use this rsync : rsync://gentoo.zugaina.org/unity-gentoo-portage
If you have a problem : ycarus(-at-)zugaina.org