If it helps, I was just digging into something like that last week. They’re a bit more tutorial/step-by-step than delving into raw code, but relatively workable for creating custom feeds.
Now that we have seen how to setup Tumblr-style posts, it would be nice to be able to segregate the Tumblr-posts category from the main feed into its own, separate feed. This would enable readers to subscribe exclusively to the Tumblr-posts feed and maybe display it in their sidebar or something.
While we’re at it, it would also be cool to be able to provide readers with a full menu of feed choices:
Everything feed: includes both the main posts and the Tumblr posts
Articles-only feed: includes only the main articles and no Tumblr stuff
Tumblr-only feed: includes only the Tumblr-style posts
Let's look at an overview of the process..