@Philipp Oeser (lichtwerk) I think this may a limitation of ED_object_pose_constraint_list. : (
The "active_pose_bone" context variable isn't accessed from buttons_context
and uses screen_context which means that it can't account for the pinning.

If I switch "active_pose_bone" and "pose_bone" like this then it works because it finds
it first in buttons_context. This seems to work fine in all of my testing, but
does it make sense to you?