Actually, I'd rather leave it to the author of the document, because it seems a tough problem to solve.
At http://markdress.org/goo.gl/zTG1q, for example, the image is linked to an absolute URL: /graphics/markdown/mt_textformat_menu.png. It's going to be a bit cumbersome to traverse through goo.gl and detect that this is actually at http://daringfireball.net/. Might be easier if authors either stuck to pure relative URLs, or full URLs with domains.
One cool little thing you could add is a javascript widget on the front page, where people could paste in their link to a markdown file, and have it automatically formatted as a Markdress link (or even redirect them to the markdress page).
Why is this getting voted down? Is there something about cloudflare i don't know about? He said he hadn't implemented caching yet so I told him an easy way to do it...
http://markdress.org/goo.gl/zTG1q