And...it's too big a feature for a single person to develop; I'll get it started, but figuring it into Maketarget is a little bit beyond me.
Example: every cervical vertebra has to bend, except for ones fused at the hip. This means it will need its own rig.
(Yoiks! Yet Another Rig!)
Another example: skull and hip sutures must be flexible, so that one can age the skeleton.
Finally, it would be a good idea an osteoarthritis set of targets, to illustrate the effects on the skeleton.