that doesn't make sense ...
NO post will be fetched if the user doesn't have permission. In other words, in showthread.php the user CANNOT see anything therefore the system will not fetch anything. The custom error message will show in it's place.
If you are still lost
(OR, I am not understanding you correctly) please post a snapshot of how/where you want this message to display ... it will help both of us. I will then tell you if it's possible.