So for a non-spherical head, for this to work, you might have the bones guiding the mouth off centre, to that the arc of their movement runs along the surface of the face of the model in question? (I think that made sense... )
I tried something similar with one of my characters recently and found that the smile action pinched in the cheeks, due to the spherical arc of the bone movement pulling the cheek in as it continued to rotate. I might try it again though, with the axis of the bone guiding say.. the right cheek , off to the left side of the head and a bit deeper in resulting in a greater arc than if it was in the middle of the head, so that when it is rotated it still follows the surface of the face.....
That may or may not work (I'll have to play around with the idea), but regardless this thread has been very useful Victor, thanks!