You might also try baking down and spawning a morph from just that dial, renaming the resulting morph, applying a match centers command to the involved actors (head, all the tongue parts, upper and lower jaw; you'll pick them from a hierarchy when you apply the command).

An even more primitive way to do it, which will simply move just the external facial/head mouth morph rather than the internal mouth parts would be to export just the head (after applying the dial at 1) as wavefront obj (all options unchecked and make sure V4 is at 0 EXCEPT for that morph, no rotations or translations applied) and reapplying the result as a renamed morph target to the head. This will just adjust the external mouth height, not the inner mouth parts.