Does not work, after extensive testing in the nexus forums and also self-testing. It works fine when unmounted, however. This is fairly important for dragon flight mounts, and the like. Just brainstorming on the approaches that people are taking to get past this roadblock.
Note: this applies to both the player and the mount, in both 1st and 3rd person modes.
I was thinking of a crazy idea translating mouse position as a function of screen resolution into an angle, but of course we don't have any functions to return that kind of info.