Not sure about all you mentioned but it definity works for Mambo 4.5.1
The URL is the important part. For Mambo it should read...
Code:
http://www.yoursite.com/index.php?option=login&op2=login&lang=english&return=/index.php
Also the password field in Mambo is called passwd not password.
I'm planning a new release that will allow for hidden fields better. But in the meantime if you having problems I'd be happy to help.
Just post the systems you trying to create a login to and a link to a working example at Mamboforge.