Has anyone proposed a human only proof of work system?
PoW needs to be hard to do but easy to verify (by a computer). There is very little left that humans are better at than computers (OCR, face and image recognition, IBM Watson [1] etc.). And probably nothing is easily computer verifiable.

One example are captchas. The ones that don't get read by OCR, simply get outsourced to children in our countries. You might say: still proof of human work, but there is no way for a computer to verify the result. On top of that that captchas don't satisfy some other requirements for PoW.

One such requirement is that the work is progress-free. Another: whatever the "puzzle" is, it must not be known beforehand as that would allow people to start the race early. This also implies there may not be one person or group of people that have access to the puzzles in advance as they would have to be trusted to not cheat (i.e. the system would not be trustless).

[1] https://en.m.wikipedia.org/wiki/Watson_(computer)