Are you doing something like this? Offhand/at first glance probably what would need to be done in BPS is to add a hook (action or filter) or shortcode. I would have to look into this further/in more depth to be able to determine the best approach. At this point I don’t see an easy way to hook into the BPS Login Security Form processing. Will post back here once we can look at this and test some things out.
http://codex.wordpress.org/Function_Reference/wp_signon
function custom_login() { $creds = array(); $creds['user_login'] = 'example'; $creds['user_password'] = 'plaintextpw'; $creds['remember'] = true; $user = wp_signon( $creds, false ); if ( is_wp_error($user) ) echo $user->get_error_message(); } // run it before the headers and cookies are sent add_action( 'after_setup_theme', 'custom_login' );