Canonical's Mir Display Server for Ubuntu Linux now has screen rotation support in the form of implementing it via its OpenGL renderer.
To take care of a several month old feature request for Mir/XMir of "screen rotation support" via this Launchpad bug
is now a screen rotation capability.
Mir Bzr revision 1345
notes, "Implement screen rotation in GLRenderer, for platforms which can't do it natively in DisplayBuffer."