ConverterInterface.php

interface Symfony\Bundle\FrameworkBundle\Request\ParamConverter\ConverterInterface
{
    function convert($value);
    function setOption($key, $option);
    function getOption($key);
}

DocumentConverter.php

class Symfony\Bundle\DoctrineMongoDBBundle\Converter\DocumentConverter implements Symfony\Bundle\FrameworkBundle\Request\ParamConverter\ConverterInterface
{
    private $dm;
    private $options;

    public function __construct(Doctrine\ODM\MongoDB\DocumentManager $dm)
    {
        $this->dm = $dm;
        $this->options = array();
    }

    function setOption($key, $option)
    {
        $this->options[$key] = $option;
    }

    public function convert($value)
    {
        return $this->dm->getRepository($this->options['class'])->find($value);
    }

}

routing.yml

get_user:
    pattern: /users/:id
    arguments:
        id: { type: doctrine_mongodb_document, class: Bundle\UserBundle\Document\User }
    defaults: { _controller: UserBundle:Users:get }

services.xml

<service id="doctrine.odm.mongodb.parameter_converter.document" class="Symfony\Bundle\DoctrineMongoDBBundle\Converter\DocumentConverter">
    <tag name="request.parameter_converter" type="doctrine_mongodb_document" />
    <argument type="service" id="doctrine.mongodb.odm.document_manager" />
</service>

UserController.php

class Bundle\UserBundle\Controller\UserController extends BaseController
{
    public function getAction(User $user = null)
    {
        if (null === $user) {
        throw new NotFoundHttpException();
        }
        // some user processing or rendering...
    }
}

I think all this can be solved if we add options to a single route that we merge into the request defaults in a _route_options key. That we can take and call setOptions on a converter?