Hello,
For your use case, it may be desirable to programmatically set the permissions of these relationship items in an onAfterPromote server event on the parent itemtype. This way, you use the built-in aras permission model to disable new relationships from being added or deleted based on the parent's life cycle state.
While the post below doesn't cover this exact use case, it does provide an example on how to programmatically update an item's permissions which you can modify to fit your needs.
http://community.aras.com/en/knowledgebase/permissions-based-item-property-values/
Chris
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Christopher Gillis
Aras Labs Software Engineer