I received an email from someone referring to him/herself as “nz”. NZ solved the challenge by creating a pseudo block header with the correct timestamp (and very low difficulty) using a python tool whose github address is included in NZ’s e-mail.
I verified that these parameters do work:
I regard this as more of a bug find than a hacking solution. My intention was that no solution would be possible unless the computed block has the correct timestamp AND a difficulty on the same order as we expect the BTC blockchain to have. Clearly this failed on the second point. So I will debug this problem and then re-issue this challenge.