This feature request is more coming from Maya and in my opinion, lack of this feature is limiting the facial rigging in blender. Currently, you can change the shape of mesh B and connect it to mesh A as shape key. Now if you change the shape of mesh B, shape of the mesh A doesn’t update in viewport.
In Maya, you can skin a mesh B to a skeleton and connect it to mesh A as shape key. Now if, you move joints on mesh B, it changes shape of the mesh A and updates it in viewport. This functionality allows the rigger to divide the face into separate parts which results in creating a very flexible setup for face. You can also apply other type of modifier on the mesh B too.
This functionality or updating the main mesh while the shape keys being updated using armature is missing from the blender. It restricts the rigger and makes it harder to create a setup using bones and shape keys which is animation friendly.