Could you solve/mitigate it with curation? Let the users be anonymous or pseudo-anonymous and keep the server signing, but add another signing field called the curator.

At first blush the curation signer looks the same as the server, but I think its subtly different in a way that is maybe effective?

A) You can re-curate or multi-curate a message, but you should (probably) keep the source community constant

B) It separates moderation concerns from on-topic concerns which is a constant struggle. Easy for a server to host many different communities/topics/communication mores and import the content stream in different ways from different curators

Who does the curation? I guess its as simple as the upvote button, but perhaps you can improve that model substantially