How about adding some javascript to the auth form so that the password field on not 'empty' to begin with?
What I though was, pre-populate the 'password' field with, say, a few spaces so it looks blank, then use a javascript OnClick() or OnSelect() - or whatever it is - function so that if the user selects the field to enter a proper password the spaces are removed.
i.e.
"username" [ Tab / click ] "password" [ Enter / click ] –->> "username"/"password" > {radius}
"pin" [ Enter / click ] –->> "pin"/" " > {radius}