There doesn’t seem to be a way in blueprints to get a reference to a constraint on a skeletal mesh that was not added by blueprint.
What I would specifically like to do is: get a constraint of my skeletal mesh, and use the “set angular orientation target” node to do what the node describes. The node input asks for a “Physics Constraint Component Object Reference”, but as far as I can tell there is no way to get a reference to one of the many “Physics Constraint Component Object(s)” from my skeletal mesh. (I am currently using the UE4 Mannequin as my skeletal mesh)