How to set db_prefix to access database on separate server.

Hi

I have been working on setting up a multisite + single sign on site, I did this by adding the following to the settings.php.

$db_url = 'mysqli://username:password@localhost/databasename';
$db_prefix = array(
    'default' => '',
    'authmap' => 'shared.',
    'role' => 'shared.',
    'sessions' => 'shared.',
    'users' => 'shared.',
);

It's working fine if all the databases are on the same server.

But is there a way of telling Drupal to connect to a different mysql server to access the authmap, role, sessions and users tables?

Thanks,
Ivan