@dee@toot.cat @noracodes Just adding that this idea goes back further (with associated lessons learned, tradeoffs, etc) into RBLs and DNSBLs in the late 90s, in case y'all haven't explored those and so folks don't rediscover/relitigate the similar pros and cons of setting this up.
Wikipedia has a decent article up (https://en.m.wikipedia.org/wiki/Domain_Name_System-based_blocklist), but https://www.dnsbl.info and other pages offer a decent entry point into further research.