This should be set per comment in your mind? Not for all comments within that entity?
My thinking would be for Fibery to treat comments like a to-many relation, then you can have many comment fields, each field can have different controlled permissions.
It’s related to this: Expose system relations in entity permissions
But if you really need it on a per comment or per thread basis, I think that makes things more complex… Both for how it works, and also the way the end user will interact with it… It means the person writing the comment needs to see a list of access templates and decide which access template should see this - per comment they make. I think that’s quite a bit ask, even if technically possible.
Would different comment fields with different access control fix your use case? How do you currently handle this issue?