Just found this multi-account fediverse client that supports ActivityPub C2S! As far as I know it’s the only client supporting the C2S, but it’s exciting that we finally have a proper client. It’d be amazing if Lemmy added support for the C2S, so users could use their lemmy account from the same app as their microblog acct.
I don’t know if AndStatus fully supports C2S as the issue about it is still open. This issue is likely also the most detailed info you’ll find on implementing C2S: https://github.com/andstatus/andstatus/issues/499