Since we now put the user-id first, before the form goes to check how to even try to auth said user, we can just fetch a strong random salt from the server in it's response "just use a simple password, this is not a fancy account". Some nice, salted PBKDF shouldn't be a problem then.

Also, there are zero-knowledge authentication systems, but they are somewhat iterative due to their probabilistic nature. They are secure in the face of an imposter authentication service, as the password (or a hash, obviously) has to be stored in the real authentication service. But it is never transmitted after this initial store, so no amount of logging or posing as an evil imposter risks disclosure.