and experts, I'm curious to know if anyone has attempted to create a Fediverse instance as an Onion Service.

I understand that server-to-server communications in ActivityPub use HTTPS POST. So I imagine the sticking point would be routing server-to-server communications over the Tor network from, say, mastodon.social to valencia.onion.

Is this a problem with a trivial solution? Or perhaps an intractable one?

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If you have a fediverse account, you can quote this note from your own instance. Search https://left-tusk.com/users/BlueDot/statuses/114468354138986736 on your instance and quote it. (Note that quoting is not supported in Mastodon.)