Well, you could do that, but you would have to remember to update that code after every upgrade. And there's no guarantee it wouldn't have obscure unintended side effects in some scenarios.

Your original idea of a nightly job to fix all the new users was probably better. reportstoid is actually a link to the role_assignments table - when a user is assigned as a manager then you need to assign them as having the staffmanager role in the user context. So for the fields in role_assignments

roleid: the id of the staffmanager role

contextid: from mdl_context, id where contextlevel=30 and instanceid= user id of the subordinate

userid: the manager id (18)

In your case managerpath in pos_assignment is easy, it'll always be /18/x where x is the user id of the subordinate

As long as the pos_assignment and role_assignment records are set up properly most functionality should work OK.