Add-on Has anybody done an add-on yet to allow the same discussion thread for multiple resources?

Out of the box, XFRM forces a one-to-one relationship between resources and their discussion threads. I have a scenario were I'd like to be able to associate the same discussion thread to multiple resources.

Has anybody done something for that yet? Thinking out loud, without looking at the DB schema, would it even be possible without breaking stuff?

Yes, it's possible without breaking stuff but I don't believe there's an add-on out there that can do it yet.