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This is similar to the rainbow table attack on password hashes. The defense against that is to use a (public) salt when computing the hash. Maybe something similar will work here, though I'm not clear about the details of your notion of "just in time" hashing. The salt just prevents precomputation of hashes.

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It would be better for the salt to be private (and revealed at the same time as the name), otherwise a miner could still try to guess at the name being reserved. Having a public salt only eliminates the possibility of precomputing many hashes, i.e. rainbow tables.

I'm not sure about the idea that a miner could guess -- the point of a cryptographic hash is to make that impractical. We're not talking about CRC32 here. If there was discussion out there about protocols that involve keeping salts private I could be persuaded that it's a valid idea.

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