Can anyone suggest a way to do this?
I downloaded the most recent X, and compiled, and it doesn't seem to
have changed from a couple of years ago.
For a very few chipsets (IIRC 2), rotate is supported as a option in the
Driver section of the XF86Config.

Is there a more general solution?
I don't care at all about accelleration, though it would be nice if some
sort of direct access were possible for programs that can output already
rotated (such as some movie players, ...).

It's for using monitors when I'm lying down.