Relative paths might work, I can't say for sure since I hadn't tested it out. I only supported an absolute path so there's no ambiguity of where the file download directory exists. If it does work it would probably work with a path relative to the drupal root folder since that would be the directory Drupal's PHP runs from (and thus attempts to open files from). By the way, if anyone needs to get the absolute path of their virtual server directory they can do so with the following PHP code.

This should spit out the absolute path of the PHP script running the code.

Edit: Ryan has confirmed that placing a path relative to the Drupal root works as well.