This is a simple node.js (pure javascript) module that's able to extract feed urls from any url. This is based on Feediscovery, but functions locally, to avoid the outrageous charges from GAE.
feender('http://blog.superfeedr.com', function(error, feeds) {
console.log(feeds[0].title); // 'Atom Feed'
console.log(feeds[0].href); // 'http://blog.superfeedr.com/atom.xml'
console.log(feeds[0].type); // 'application/atom+xml'
console.log(feeds[0].rel); // 'alternate'
});
Works great with Superfeedr!