I'd be concerned about being accused of/blocked for crypto-jacking. That said, I'd like to see a proof-of-work based captcha that donates any earnings to charity as cryptocurrency.

I came across hashcash.io before, previous discussion: https://news.ycombinator.com/item?id=7944540

Alternatively I could write something dead simple from scratch that looks for sha collisions to burn a few cycles. Would this be less dubious?