I think this brushes over the licensing issues a little too quickly.
Does transcluding a foreign CC-BY-SA template mean the destination page has to be CC-BY-SA (probably, but IANAL)? I would say certainly if subst: is used (but still IANAL).
If so, even in the Wikimedia context, there are some possible issues. E.g. English Wikinews is under CC-BY 2.5, not CC-BY-SA 3.0. CC-BY-SA content can't be imported into a CC-BY wiki.
This doesn't block the feature. But depending on the answers to these questions, we might need some kind of metadata on the repositories. We could try to check simple known cases (e.g. CC-BY-SA on both, or CC-BY on the source, CC-BY-SA on the destination), and allow the admin to override it in case we got it wrong or they received different legal advice.
I don't think we should just expect each sysadmin to solve this properly on their own.