What a wonderful character and facial rig test! (If only my tests turned out so well.) WOW!
And thanks for sharing some of the details about your facial rig. I noticed that you had a pose slider labeled "Corner_L_IO". I assume that "IO" stands for "In-Out", as "LR" stands for "Left-Right" and "UD" for "Up-Down". Also (assuming that the bones' roll handles point either up or down) In-Out involves scaling a bone in its Z-axis, Left-Right involves rotating a bone around its Y-axis, and Up-Down involves rotating a bone around its X-axis. Correct?
In trying to play with a test rig based on your facial rig, I noticed that one could either scale or translate the bone in its Z-axis. Translating moved the center of the sphere described by the bone as its radius. This seems to have some interesting possibilities, but I assume that you used scaling since you do not want your sphere's center to move. Correct?
You mentioned that you found that using a single bone for both Left-Right and Up-Down rotations caused some unspecified problems. Assuming I'm on track (which may be a bad assumption on my part), did you find any issues regarding which bone you scaled (i.e., the LR bone or the UD bone)?
I was also wondering in regards to your character's arm rig whether you were going with a strictly FK rig? IK? or one that can select for either?
Thanks in advance for any answers you may provide. It is greatly appreciated. I've been struggling with the rigging of my character's mouth for a few months now and have been encouraged by seeing your facial rig. Although my character's head isn't spherical, it appears I can adapt your rig to my situation.
Thanks again,
Dean