@J12t I implemented something like this in a federated private-post social media system I wrote-- in order to hide one's friend list, all responses to a post are only sent to the OP's device, which then fans it out to all recipients of the original post. Responses can be anonymous (except to the OP).

The main problem with this is that the OP's device has to be on and able to process incoming responses, or else the conversation will pause until the OP comes back online. It's only really a problem on mobile, and I'd guess it's solvable with push notifications to a background process (which is what my app does). However, background processes are problematic on mobile, as they can be terminated by the OS without warning. I think there's a way to mitigate this, but that's around when my development stopped.

Another solution is to have a trusted always-on computer for each posting user, to do that reply processing. However, that can be a barrier for many mobile users, but you have a different use case.