Gonzalo Ayuso has a new post that can help you protect certain files inside of a public folder by combining mod_rewrite and PHP.

Here's the problem. We have a legacy application (or a WordPress blog for the example) and we want to protect the access to the application according to our corporate single sign on. We can create a plug-in in WordPress to ensure only our single sign-on’s session cookie is activated.

In his example, he shows the handling of an uploaded file and a plugin that can be used to protect parts of the site based on session information. Unfortunately, by itself, this doesn't prevent the direct access of the file. His trick is to route all file access back through a central "media.php" script that fetches it from a file location (could even be outside the docroot). The routing to the PHP is handled via mod_rewrite and the code checks the permissions on the current user's session for access.