djgreg,
There are multiple problems. First is that mod_rewrite's work best in the virtualhost directives not in .htaccess files. Second, why rewrite the static links back to the dynamic ones? Why not just remove the static page builder module? Third, %{REQUEST_FILENAME} is the full path to the files, which your condition does not check for.
This works (I set this up in your VirtualHost for forum in /etc/httpd/conf/httpd.conf):
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} .*/forum([0-9]+).html
RewriteRule (.*) /viewforum.php?f=%1
It's not perfect, but it does work and it should hopefully show you the path to follow to find the best solution to this problem.