Copy Pose from Mesh works fine when you’ve got the same skeleton or same skeleton+a bunch of extra bones. It would be nice however if it would work if the hierarchy wasn’t the same, just copying the offsets of the bones anyway.
For example if you created a hair rig with a root bone and a head bone (called head) only. If you used Copy Pose from Mesh on that targeting a normal mannequin style rig nothing would happen because lots of bones are missing (pelvis, spine, neck and so on). Ideally it would still work by copying the bone transforms at the point where it exists in the target mesh skeleton hierarchy. You could then attach whole rigs to certain bones which seems like it would be useful. Especially since you can add bones when using Copy Pose from Mesh.