I do not have an exact solution to you problem, but maybe I can point you to a direction which might help you.
About the philosophical argument, I mean if relationships are own entities, you do not change the items those refer to.
But if the relationship is owned by those items you change those owning item if you change the link. Then there is always the question who is the owner and when access rights come in place the users get lost.
Maybe the Asset Service can help, you but I cannot exactly explain why. I can just give you my observations.
I have a library intended to be used by many projects. This library I maintain in a separate repository to have it clean (no references to any usage).
This library I import in a concrete project via EA Asset feature. In the project I lock the asset package for any user. The project related stuff I put in other packages having user group locks.
When modeling in my project related packages I can link from my project related stuff to the read only assets in any direction (from my point of view essential when using assets).
But even more, I can create links between asset elements as well. What from my point of view changes the asset and should therefore not be there, but maybe it is the door to solve your problem (as long as the door is not closed by a bugfix).
When updating to a new asset version I do the following on the projects using the asset library:
1. Backup all somehow (just if disaster happens)
2. Baseline all project related stuff (excluding asset library).
3. Import new assert library version
4. Restore baseline
After step 3, some (but not all) project items to asset items relationships are lost (presumably those owned by the “read only” asset items because the new asset version do not have those anyway).
After step 4, those lost relationships are there again, because presumably the baseline knows those missed but “not owned” relationships as well.
Links created between assets are lost in any case (not a problem in my scenario).
Maybe if you handle your elements as assets and your diagrams as “project related” it might work for you. The question is how long?!?