I think I have hit on a potentially useful trick for rigging eyes the way I want (for a couple reasons I’d rather not use track-to)… But in order to get this to work right for all head poses, I would need to be able to constrain an object’s rotation to the sum of two other rotations. Can this be done?
Turn the 2 red cubes on the z axis and the blue cube will rotate the sum of the 2 blue keys rotation. You can set up the other axis as well if you wish.