I believe that to hide information in a post you will need a custom plugin. I’m sure that you need a plugin; it’s possible that one exists that I don’t know about.
If you don’t have a $300-1000 budget for such a plugin, then the easiest way to solve what I think is your problem is to have a public topic that says "You want something great? Well, you have to do something to be able to have access to link_to_topic_in_private_category.