How can I modify the Appian login page?

You would need to modify the Appian product files to achieve this and Appian doesn't recommend to do so. Better you could think to write a servlet plugin which will hold all your custom logic to check loggedInUser authentication and other files such as html, JS etc and deploy it to the plugin folder. Have a look at Developer site: forum.appian.com/.../Developer_Guide.html and API : forum.appian.com/.../

You would need to modify the Appian product files to achieve this and Appian doesn't recommend to do so. Better you could think to write a servlet plugin which will hold all your custom logic to check loggedInUser authentication and other files such as html, JS etc and deploy it to the plugin folder. Have a look at Developer site: forum.appian.com/.../Developer_Guide.html and API : forum.appian.com/.../