Yes, I have cobbled together a very raw, but functional proxy here if you'd like to see/audit/play with: https://github.com/relaytools/interceptor
https://github.com/relaytools/interceptor-proxy

Decided that I don't mind pushing ugly code early and often as I work through what it means to have auth enabled.. (and the various other relay modes it will eventually be supporting, like just protecting DMs or etc).