Could you still achieve your deployment goals with a Gateway/WAF setup that proxies the user-facing endpoints? The only issue I'm presently aware of with this setup is https://github.com/keratin/authn-server/issues/8.
I see the motivation for sending user passwords straight to AuthN, but I do wonder if on balance the dangers of correctly configuring a shared public/private service don't outweigh the dangers of relaying the password through your front-end (which has to be able to access private endpoints on the AuthN service anyway). If you really didn't want your front-end to touch passwords, you could have a tiny sidecar service that only exposes the public endpoints and speaks GRPC to AuthN.
Anyway, this is well into debatable "matter of opinion" territory, and I don't want to waste your time. Thanks for publishing AuthN - I'll keep a close watch as you progress.