But whispers are available only to staff, and you need the author of the post to be able to see it. And it could be any post, right? (Maybe any post in a certain category).
A plugin could add a custom field that’s added to the serializer only for the post owner and staff. Then you’d have both of those separate posts in one place, at least.
If what you really want is a [HIDE] BBCode, then . . . no. The raw post is still visible to everyone who can see the post. It might be possible to encrypt the hidden text in the raw post and then somehow contrive to decript it when an admin or the owner sees it. Seems tricky, though. Or maybe you’d just dupe the whole raw post when it’s saved and then do something to use it when you edit or display the text for the owner.