Nip-29 enables this. And you can set any terms you like btw. You can make people ask permission, set a price, have then post "I like potatoes" in their public feed, etc...

It's just a matter of when a kind1 client implements this. Which is tbh slightly complicated by them having to fetch those private notes from specific relays. 

Highlighter and other non kind1 focused clients are already set up to do this.