i have some ideas but i don't know if there isn't any easily way to do that.
In the login operation of the UserModule, you have an "url" variable. So, you can call this operation with the id of the html page you want to show (like /index?module=user&op=login&url=/toto.html)
You can modify the loginBlock to do that but i don't know howto.
you can modify the login operation. You can get the group of the user and redirect to a file like /GROUP_DEFAULT.html page.
So, you have to store in the database a GROUP_DEFAULT page for each groups.