I suspect the "mobile version" of the site isn't URL controlled, but is rather a "theme" consisting of the relevant html/css optimized for mobile/small screen devices which is activated based on the content of the user agent string (passed from the client browser to the server which identifies the device name) somewhere in the forum software.
Anyone with some knowledge of php web development should know how to whack in a php page to make it url controlled. I would myself but I find the whole php development process rather tedious plus I don't have access to the web server