Hi there, I made webmention.io. I haven't updated the readme or home page yet, but I recently added a "web hook" feature which is similar to what you're talking about with Trackbacks. The service will receive and verify the webmention, parse the page, and then send a post request with the contents of the comment. The format it sends looks like this http://indiewebcamp.com/jf2#Example
So, it's not quite trackback, but hopefully it's more useful!