I don't understand the purpose of this. What's the point of pointing people to the Bitcoin blockchain to look for public keys? Ultimately, the authority that puts the relevant information into the OP_RETURN output decides what people get. How will users know they get the right information unless a) Urbit signs the relevant data somehow, or b) points them towards a specific Bitcoin transaction ID? In either case I don't see what the advantage is of putting something in a blockchain, when that something comes from a central source of truth -- why not just consume this information directly from the central source of truth?
> And best of all, you can get your Urbit star or planet entirely through the blockchain, without interacting with any centralized database or payment mechanism.
Great! So all the slowness of a blockchain without any of the advantages?
To be clear, I'm fully aware of how a blockchain is useful for governing internal data (e.g. Bitcoin blocks excluding double spends). But I don't see the point of third parties putting data into blockchains, and pointing other people there to retrieve it, unless we're talking about some sort of proof-of-existence (which utilizes the timestamping feature of a blockchain).