I think “shortcode” is just another term for BB-code.
And the feature he wants is a bbcode that emits
- if there is no parameter and the current user is registered and activated or if there is a parameter and the current user has written at least this many posts: its contents,
- in all other cases: nothing.
Since Discourse caches the cooked post, a Discourse-specific version of this feature would either be significantly harder to write (replace tag content with clickable placeholder, fill in hidden content async on click with permission check on server), or would be laughably insecure (content present in cooked post, hidden via CSS, revealed by JS).