I think the easiest way would be to do referrer-checking. Link to the "protected" page only from the pages that only logged in ppl can see. Then, in the .htaccess file, restrict the referrer to the "protected" forum page.
The referrer can be spoofed if someone really cares enough, but it's not a trivial task, and most ppl won't care enuf to go to the trouble.
The .htaccess stuff can be found in the apache manual for your apache version.
Alternatively, you could create a new page (inside your forum's page root that it can protect via its own session checking) that include()'s the protected pages from somewhere outside your htdocs root of your webserver.