I don´t have an answer, but I want to share a few thoughts.
As you already noticed the lock column is only missing in null-relationships. When you edit an Item and it´s null relationships the normal way (via Form) and watch the related tables (e.g Part Goal) you will noticed that the NULL relationships are not locked while editing the parent item. Somehow NULL-relationships are handled a different way then regular relationships. Changes in NULL relationships seems to bypass the regular lock-edit-unlock routine.
By default, Innovator assumes that you only change null-relationship from a parent item and not change them directly. So you normally shouldn´t have problems with forgotten locked items.
I assume adding the lock column in a null-relationship requires the change of codetree files. I don´t expect this one to become easy.
I hope somebody can share more helpful insights to this one.
