I'm not sure if this would help, but perhaps attaching the lower jaw to the torso bone would help make the models appear more lively when animated. This was done for a few of the larger fantasy units in Blue Lotus. It makes the units sort of move their mouths slighly when walking or or attacking.

While it looks great in BL, I'm not sure how it would work out with the medieval 2 Total War skeleton. It might be worth a try though.