The solution involves a bit of hacking via a list comprehension of ids,
since related_name='+' has been set for contributor and unregistered_author
fields of PublicationAuthorsTable.

The previous version returned the wrong sort of objects, i.e.

return self.publicationauthorstable_set.select_related(
   'contributor').order_by('contributor__user__last_name')

returned PublicationAuthorsTable objects instead of Contributors