@julian I don't typically deal with abuse online, so maybe not the best person to answer this, but personally, I'd be fine if the reply was hidden from me and only lived on the abuser's server.
I'm guessing third-party servers that implement reply controls also wouldn't see the reply? I guess that should be sufficient.
Obviously you can't prevent people from posting stuff on their own website/blog/etc, but there have to be ways to limit the reach.