I’m not sure that the implementation is the real issue. Mastodon posts can be exported already. Writing the import code wouldn’t be that hard. But why would a Mastodon server’s admins want to let a brand new login post a big pile of backdated posts all at once? It seems spammy and the moderation workload scales with the number of posts.

Maybe a dedicated archive would be better. It would be a matter of generating a static website from the export. Nobody else would need to moderate it, because it’s not their website.