It is easiest to have the login form post to the login page, but you can add in the Authenticate user server behavior to any page. So you can have this server behavior on the announce page and have the small login form post to this page.
I would not suggest having only a password to login, this is not very secure. How do you know to match up the password? You will need to know who the user is and the password to check on both of these fields. If you are assuming the username and just matching the password you could have a problem if more than one user has the same password. I would suggest keeping it traditional and using a username and password.
