I think that applying a captcha to curation would have the opposite effect to what you intend, reducing competition, leaving whale votes with more influence than they have now, and creating even more disaffected authors. But, decentralized captcha is a very creative idea that might well have other uses, so I commend your ingenuity.

I did a quick search and found one proposed system described here - https://grez911.github.io/captcha.html. No idea whether it's feasible to implement such a thing on the steem block chain or not.