Detect in Symfony2 TypeExtension if a type is a prototype root

Form type extension problem

I'm writing a FormExtension that apply to all form type (experimenting with angularjs... another story :), but I need to detect at build time if a FormType is used as prototype root.

class MyFormTypeExtension extends AbstractTypeExtension
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        if (true /* === $iAmPrototypeRoot*/) {
            $builder->setAttribute('i_need', 'magic');
        }
    }
}

My problem is that I need to set some attributes that I need later, but setAttribute is (or seems to be) available only at build time.

So far...

I've checked the source code for CollectionType and found this:

class CollectionType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        if ($options['allow_add'] && $options['prototype']) {
            $prototype = $builder->create($options['prototype_name'], $options['type'], array_replace(array(
                'label' => $options['prototype_name'].'label__',
            ), $options['options']));
            $builder->setAttribute('prototype', $prototype->getForm());
        }

        // [...]
    }
}

So I know that I can override this template and add a "fake" option is_prototype to the options, but this is a bit hackish since I'm overriding (not extending) the default CollectionType. Example:

class CollectionType extends AbstractType
{
    public function buildForm(FormBuilderInterface $builder, array $options)
    {
        if ($options['allow_add'] && $options['prototype']) {
            $prototype = $builder->create($options['prototype_name'], $options['type'], array_replace(array(
                'label' => $options['prototype_name'].'label__',
            ), $options['options'], array(
                'is_prototype' => true
            )));
            $builder->setAttribute('prototype', $prototype->getForm());
        }

        // [...]
    }
}

Which solution?

Up 'til now, the best solution I see is to add the 'is_prototype' to the collection option option (no pun :); but this should persist if the user override the option (maybe a Normalizer?)