JMSSerializerBundle, MongoDB, and XML Metadata

Question

I've been wracking my brain over this all night it seems. Every time I try to serialize the results from a document query, I receive a result of bool(false). Presumably this means that the serialization is failing (great! Perhaps you'd be willing to tell me why you're failing? No? didn't think so.)

I have told the JMSSerializerBundle where my FOSUserBundle mappings are held, e.g.:

jms_serializer:
    metadata:
        file_cache:
            dir: "%kernel.cache_dir%/serializer"
        auto_detection: true
        directories:
            FOSUserBundle:
                namespace_prefix: "FOS\\UserBundle"
                path: "%kernel.root_dir%/config/serializer/fosuser"

I've defined a very simple XML serializer metadata file, e.g. Model.User.xml:

<?xml version="1.0" encoding="UTF-8" ?>
<serializer>
    <class name="FOS\UserBundle\Model\User" xml-root-name="user" exclusion-policy="all">
        <property name="username" type="string" expose="true" />
    </class>
</serializer>

And, in a test controller I'm manually invoking the serializer before I attempt to do anything more sophisticated, e.g.:

class DefaultController extends Controller
{
    public function serializeAction()
    {
        $userManager = // get the user manager
        $users = $userManager->findUsers();         
        $serializer = $this->get('jms_serializer');
        foreach ($users as $id => $user) {
            Debug::dump('Processing user: '.$user->getUsername());
            $result = $serializer->serialize($user, 'json');
            Debug::dump($result);
        }
        /*return a response */
    }
}

The result? Always bool(false).

However, if I manually construct a new User object, the serialization works fine, e.g.:

class DefaultController extends Controller
{
    public function serializeAction()
    {
        $userManager = // get the user manager
        $users = $userManager->findUsers();         
        $serializer = $this->get('jms_serializer');

        $user = new User();
        $user->setUsername('testusername');
        Debug::dump('Processing user: '.$user->getUsername());
        $result = $serializer->serialize($user, 'json');
        Debug::dump($result);
        /*return a response */
    }
}

Any thoughts? I'm about ready to lose my mind over this.

EDIT: Also on further reflection, it is worth noting that it doesn't look like my serialization metadata is being respected. As shown above I'm only exposing the username, but when I create a new User in the controller and serialize it I see more than just that exposed, I'm seeing: username, enabled, locked, expired, roles, and credentials_expired. I suspect that I would be seeing every field, if it had non-null data in it.

EDIT (2): I've seen references to using the DoctrineObjectConstructor over the default object constructor for the serializer; and took the steps identified in this answer hoping that they might resolve my issue. Alas, they didn't (looks like that answer and question were more for deserialization though any way) and the result of a bool(false) persists.

EDIT (3): The issue wherein the serialization metadata was not being picked up, appears to be due to the mixing of auto_detection and the explicit declaration of directories, as illustrated in another question I've asked: JMSSerializerBundle mixing auto detection and explicit directories?