I'm working on a site for somebody in which they have an image portfolio and wanted to create a MediaRSS feed so that the portfolio could be viewed in CoolIris. Searching the forums I could find one post asking how to do it and the response was to extend RSSFeed.
This was just thrown together quickly and is probably not the best way to do things, nor is it currently set up for anything but images, but I thought I'd post it nontheless in case somebody else could use it as a reference..
The PHP files all go into mysite/code, and the .ss is in my {themedir}/templates.