Currently, a subdirectory of the root can only be mapped to a relative directory of the main project settings. The project I'm working on has a root project path, but several of the subdirectories represent virtual hosts. The root of the project doesn't actually have a URL at all. This allows various aspects of the site to utilize a single codebase without duplication.
/root/www/ -> http://www.example.com/
/root/ui/ -> http://ui.example.com/