So, I think the logic is, if Unity Greeter interprets a prompt from PAM as a username or password entry it displays those as it does currently. If any other prompt is detected, it uses a "continue" button, then slides to the "complex authentication dialog" which can ask those questions / show prompts.
So, I think the logic is, if Unity Greeter interprets a prompt from PAM as a username or password entry it displays those as it does currently. If any other prompt is detected, it uses a "continue" button, then slides to the "complex authentication dialog" which can ask those questions / show prompts.