@Gargron @seachanger I won't profess to be as much of an activityPub expert as you, but would it be possible to amend 7.1.2 Forwarding from Inbox in such a way as to allow you to control which replies are forwarded, and maybe add a signature? Then you would have a way of knowing which replies were "authorized" by the poster, and clients could chose to filter only "authorized" replies using a clientside setting. Could potentially also be used to moderate replies, via a revocation.
Wouldn't prevent the posting, but it would allow you to control the visibility a lot more, which I think is what most of the people asking for these tools are hoping for. You could see the unfiltered replies if you want to, but I think people would prefer not to by default.