But I think, it would need to be decentralized, to avoid single DB somewhere that could be abused and manipulated, or lost.
But if you have to use a tool to read the graph from the web yourself, and people have to upload recommendations to their own server, and have them directly associated with their domain, that may encourage saner behavior, too.
If someone wants to make a centralized public search for this graph, so that people who don't see value in building their own index/ranking could search for content too, that would be possible.
But the key aspect would need to be the distributed nature. It would not need any cryptography either, just a bunch of text files at the simplest level.
The graph would be out there on web servers, the creativity would be in creating the tools to scrape it, rank stuff, and searching through it. And that could even be done in some incremental way (without the need to scrape it all at once).