Split Body Animation framework

Hi Everyone,

I'm trying to plan a framework for playing split body animations on our in-game units.
We need our units to be split at the waist and some units will be mounted on creatures. We'd also like our waist bone to be procedurally aimed so units can play their attack animations in their upper body with their torsos pointed arbitrarily to the left or right.

Our game will have maybe 50-100 assorted such units on screen.

I plan to have our animator insert an unanimated/unskinned bone somewhere in the waist for me to procedurally rotate. It won't be skinned to any vertex so I'm hoping the geometry won't start pinching if I rotate the bone too much. That's how I'll control the torso twisting.

As for the split body portion, I want the units to have a number of base states in the lower body (idle, walking, attacking), as well as a corresponding states in the upper body. If the unit is walking and I want to play an attack animation in the upper body only, I would like the lower body to be undisturbed in playing his existing walk.
However, if the unit was standing idle, I would like his attack animation to play on his whole body. If he then starts walking during his attack, I would like only his lower body to play the walk animation.

I understand this would be done with a new layer (for upper body) with an appropriate avatar mask, but how would the state machine transitions in each layer look? Would I sync my 'upper body' layer to the lower body? Would I need separate Bool parameters to control the upper body and lower body state transitions? If I play an 'attack' animation in the upper body only, how would I gracefully blend that portion of the body back to match what's playing in the lower body, once that animation is done?

Thanks in advance,
Jeff