Since Mediatomb already has an API for adding virtual containers and proxy eternal content, how about going on step further. How about an API for adding remote locations, complete with virtual containers and items.
We add a new container from the web interface called RemoteContainer, and point it to an http link. When clients request browsing this container, mediatomb makes an HTTP GET request to our server which sends back a list of its contents (containers, items, or both) which it can then use to send back the renderer. This will make it easy to add your friend's content.