Hey this is a write-up for an IK shoulder girdle joint chain.
I made the shoulder girdle an IK chain just like the arm. Clavicle->Acromion->Scapula is treated like Shoulder->Elbow->Wrist.
The only thing "outside" it is the point constraint from the Acromion joint to the Shoulder joint of the arm IK.
I placed the shoulder girdle IK chain completely flat to be coplanar, and it hits at the root of the clavicle, Humerus, and sort of center of the top of the Scapular spine.