The attached XSL will convert a MM file to TSK, the format used by TaskCoach ( http://www.taskcoach.org ), a popular todo manager (FOSS).
At the moment it doesn't export richtext nodes properly -- surely you already have a good way to "flatten" them, but I can't find it. TaskCoach doesn't handle HTML, afaik.
TSK files require a date-time value in the "startdate" attribute of tasks; it has to be set in the past for nodes to be seen as "active". I don't know if your XSL parser handles XSLT 2.0 (many don't), so I used a 1.0 extension available online in order to do that -- I add the current timestamp, and since more than a second will always pass between the file being saved and the export being opened in TaskCoach, it seems to work.
Let me know if you need anything else.
converts MM files to TaskCoach format