You could do the guest account thing and on start up, check to see if they're using webDirect and if not, re-Login.
You can never keychain a password this way, which I think it a feature, though others may not.
How do you plan on presenting a login/password screen given that there is no way to (correctly) mask a password in WebDirect? (Given the lack of a masked password field type in anything but a Show Custom Dialog script step...)
FYI, you can script the login of a user without having them worry about seeing the username/password field by using the Re-Login script step.