I really needed this service, so I set one up at http://lorem-rss.herokuapp.com/
The documentation is at that link, as well as in the GitHub repository below. You can tweak it using GET parameters to update every N seconds, minutes, hours, days, months or years. But I haven’t yet tested it comprehensively so I’m not sure if the RSS validates well across different parsers. YMMV.
The code is on GitHub at https://github.com/mbertolacci/lorem-rss