I ended up doing this for the comments table:
ALTER TABLE django_comments DROP CONSTRAINT django_comments_user_id_fkey;
ALTER TABLE django_comments ADD CONSTRAINT django_comments_user_id_fkey FOREIGN KEY (user_id) REFERENCES membership_person(id) DEFERRABLE INITIALLY DEFERRED;
And similar for the comment flags table:
ALTER TABLE django_comment_flags DROP CONSTRAINT django_comment_flags_user_id_fkey;
ALTER TABLE django_comment_flags ADD CONSTRAINT django_comment_flags_user_id_fkey FOREIGN KEY (user_id) REFERENCES membership_person(id) DEFERRABLE INITIALLY DEFERRED;
Hopefully that does the job and doesn't cause any problems. *crosses fingers*