We have a use case too. We’re using PMs to a group as a means of private help requests, but we are encouraging users to allow us to make their request public. We prefer public help requests then more people can benefit from the assistance.
Ideally we’d copy the OP help request, and selected replies to a new public topic, leaving out the auto-responder message and replies where we ask if we can make it public etc.
As it is we will have to make the PM topic a full topic in a private category, delete the replies we don’t want to show, and then move to a public area.