Not that I'm advocating this as a good authentication solution, but the worst sorts of exposures you list (i.e. the ones that aren't restricted to the authicate

Not that I'm advocating this as a good authentication solution, but the worst sorts of exposures you list (i.e. the ones that aren't restricted to the authicatee's device or the authenticator's server) don't apply if you use SSL. Which of course you do.