Well, you can already manually backfill one post at a time if you copy-paste the URL of a post into the search box on your server.
For example, if you copy paste the URL of a PeerTube video in a search box on Mastodon, that video will become visible to that server (if it wasn't already).
There's no limit on manual backfilling as far as I know. It's only a question of what kind of automatic backfilling there could be, because most don't want to manually backfill posts one at a time.