Some code that make you want to use Zend\Registry

gistfile1.aw

// FIRST :

class Register extends Form
{
    /**
     * Init the form
     */
    public function init()
    {
        $this->setHydrator(new ClassMethodsHydrator())
             ->setInputFilter(new InputFilter());

        $this->add(array(
            'type' => 'Member\Form\Type\Student',
            'options' => array(
                'use_as_base_fieldset' => true
            )
        ));
    }
}

// WHERE MEMBER\FORM\TYPE\STUDENT :

class Student extends AbstractUser
{
    public function __construct()
    {
        parent::__construct('student');

        $this->setHydrator(new ClassMethodsHydrator())
             ->setObject(new StudentEntity());

        $this->add(array(
            'type' => 'Member\Form\Type\StudentProfile'
        ));
    }
}

// WHERE MEMBER\FORM\TYPE\ABSTRACTUSER IS :

class AbstractUser extends Fieldset implements InputFilterProviderInterface
{
    /**
     * Constructor
     */
    public function __construct($name)
    {
        parent::__construct($name);

        // LOTS OF ELEMENTS

        $this->add(array(
            'type' => 'Zend\Form\Element\Email',
            'name' => 'email',
            'options' => array(
                'label' => 'Adresse e-mail'
            ),
            'attributes' => array(
                'required'  => 'required',
                'maxlength' => 128
            )
        ));
    }

    /**
     * @return array
     */
    public function getInputFilterSpecification()
    {
        return array(
            //
            //
            // NOOO NEEED SM HEREE TO GET ENTITY MANAGER :(...
            //
            //          
            'email' => array_merge_recursive(
                $this->elements['email']->getInputSpecification(),
                array(
                    'validators' => array(
                        array(
                            'name'    => 'StringLength',
                            'options' => array(
                                'max'      => 128,
                                'encoding' => 'utf-8'
                            )
                        ),
                        /*array(
                            'name'    => 'DoctrineModule\Validation\NoEntityExists',
                            'options' => array(
                                'em'       => $this->getEntityManager(),
                                'entity'   => 'Member\Entity\AbstractUser',
                                'property' => 'email'
                            )
                        )*/
                    )
            ))
        );
    }
}