You may be trolling, but that "exercise for the reader" does not have a known solution. Anyone who found one may wish to keep it secret to get rich on Bitcoin mining...
There are two layers of entropy in what I'm looking at, but I only got like two hours of sleep last night.
There's the entropy of the password from which the hash is generated, which is clearly what you're addressing.
But in the game I'm seeing, the hash itself is unknown but the game gives you feedback on the contents. So pinning characters of the hash cuts down on that search space. Then there's still the matter of finding a plaintext that hashes to that value, which as you've said should evade this sort of analysis.
He didn't say you could "pin" the hash. He said you could eliminate all hashes, that don't contain the positions known, and just enumerate those which contain the known positions (perhaps by bruteforce), therefore reducing the search-space. It'd still be ridiculously expensive, of course (as in, implausible to compute in this universe). Unless I'm misunderstanding something here.