How about this: Duplicates of the old posts are created on a new host using some import/export mechanic, but they also contain a cryptographic signature proving ownership of the old posts (utilizing some secret only the original poster would have access to).

When a reference to a post becomes invalid because it's underlying host is gone, the other host holding those references tries to find new duplicate posts with the correct signature, and if it finds them, updates the broken old references to point to these new duplicates. If it doesn't find any duplicates with a matching signature, it leaves the references broken and has some cooldown before it tries to fix them again.

Can we please do content addressing one day?