This is really cool, and could be a great Discovery tool. I’ve had an idea kicking around my head for something similar; a directory of sorts, where sites opt-in via webmention. Any p-category tags in the webmention post would be used to classify the listing in the directory, allowing people to find sites by groupings. Add in some sort of Technorati-style search for an extra layer of power.
I’m teaching myself Laravel at the moment… this would make a good first project to prototype, I think.