Captcha Quicky
Synchronized captcha filling could overwhelm non-human attackers which can then consist only of a fraction of captchas done. This would make the captchas a useful source of information for a OnePerID,
Brain from: https://openclipart.org/detail/140701/brain-by-trubinial-guru
It is of course unclear if captchas can be effective; being human-easy enough and computer-hard enough. Also when an image needs challenging, computing the image generation could be expensive on the consensus system. (Possibly use cheaper ones like hanging blocks)