On the "jaw and finger" issue, that is what gives support to the idea that your body grows as a whole. The muscles in my jaw are more prominant than they were when I was much smaller. It isn't like I was chewing weights over the years.
In fact, MOST bodybuilders have larger facial muscles the bigger they get. Dexter Jackson is one whose face has changed drastically from when he first started competing. It gets wider along with his neck.
As far as whether you "train" them directly for growth, I agree that you would probably either see fatigue or injury issues before you actually just built up your jaw muscles alone. It sounds like a TMD waiting to happen.