I haven't tested this but I believe you can populate an application cache using an iframe. View the source of this page to see an example of this in action.
http://appcache-demo.s3-website-us-east-1.amazonaws.com/offline-iframe/
So, for your scenario, you could programmatically append an iframe with a link to your manifest only once login is complete. You could then include your login page as part of that manifest so future visits from users who had successfully logged in would work even when offline.