Given that the admin of any instance with a single approved follower can see the contents of the community, this idea feels like placebo privacy. The false sense of privacy could be counterproductive.
The only way I can think to federate with something resembling true privacy would be to use PGP or similar. Encrypt the data with the user's private key, send it to and store it on remote instances encrypted ,and decrypted in JS on the user's computer. That would require users to mange private keys which they would no doubt lose, and be a lot of work for a pretty niche feature.