Which bone to declare in layered blend?

I have a skeletal setup using mocap, part of the bone setup goes like this:

  • spine03
    – clavicle_l
    – neck_01
    – clavicle_r

clavicle_l is the root of the entire left arm, neck_01 is the root of the head, and clavicle_r is the root of the entire right arm.

Now it was easy for me to separate the upper body and lower body, just by using the spine as the bone in the layered blend that separates the upper body animations with the lower body animations. The problem is how to separate the left arm, right arm, and neck / head animations since neither is the root of the other.

For example, how do I set up blend slots so that the animation will only affect the head and right arm? If I put the blend pose by bone on spine03, the left arm moves which I don’t want. I tried making two slots, one for the head and one for the right arm, but when I try to combine them afterwards my character becomes a warped distorted monster that’s twice as tall. Conversely, is there a way to just tell the animation blueprint to ignore the left arm entirely?