When I was interested in a full 3D game engine for the OG Xbox, I wanted to do a simple technique that Conker Live and Reloaded did with facial expressions. The mouths where 3D, but the eyes where 2D textures with 2 stages. It's not shape group based (I think) but uses bones for the mouths and textures for eyes like the outside and the inside of them.
There might be an 8 for this. When I finish my own skinned mesh renderer and animator then I will do this. Seems to be the best looking way to me.
Lipsync Oh Wow.mp4