I have been delaying this functionality because I expected it to be hard to do, then... it was very easy! Now you can create "metafeeds" based on queries. Here's the first example: "Post.important==True".
Of course, beyond a set of standard metafeeds, a UI to allow the user to create them will be hard but at least the basic functionality is there (BTW, it's 15 (fifteen!) lines of code)