Using setSlaveOkay we can tell mongo driver that it is allowed to connect to slave servers and execute queries on them. Functionality is explained here http://php.net/manual/en/mongo.queries.php php driver allows this to be set on every level connection, db, collection...
I suggest setting it on connection level and make it configurable with connection settings like bellow.
$conf['mongodb_connections'] = array(
// Connection name/alias
'default' => array(
'host' => 'slave1',
// Database name
'db' => 'drupal_default',
'slave_ok' => TRUE,
We would then set setSlaveOkay to true in mongodb function. I'm working on a patch.