The ROKU is an XD
I will try to play with My Media,
A simple personal media server and channel for streaming your music, video, and photo collection to your Roku
http://forums.roku.com/viewtopic.php?t=25955
From the link above, it looks like it is able to stream from a "web server", and it gives a guide on how to install something like Apache. I'm not quite sure how this works but I guess you put your media files in the web server's root directory and the folder is then accessible by HTTP, i.e.
http://yourservername/mediafiles/music....
In other words, it doesn't use DLNA as its streaming protocol, therefore it won't work with MC.
However, since MC does contain a web server for its WebRemote functionality, you could investigate putting your media files in a subdirectory of the MC web server (the pathname must be discoverable somehow, the path for customisable WebRemote skins is known...) and seeing if you can access those files by http.