Please create an ebuild for mythtv-backend or edit the mythtv-ebuild and
make a USE-Flag so you can install only the backend of mythtv.
This could be done by using qt-embedded instead of qt as dependency.
So, you don't need X support.
Not going to happen. The code is digusting as it is, supporting a setup like this won't happen. You can try to use the "backendonly" USE flag but I won't support it. If it doesn't work, you're on your own.