The question here is when to invoke the closure in the "empty_data" option, which is used now to pass field values to the constructor of the entity. Right now, the closure is invoked after submitting the fields, but before mapping them to the data:

1. Submit fields
2. Invoke "empty_data" closure to create entity
3. Map fields into entity

The solution here would be to submit unmapped fields first:

1. Submit unmapped fields
2. Invoke "empty_data" closure, may access data of unmapped fields
3. Submit mapped fields (maps into entity automatically)

Another side effect of this workflow is that the initial value of the added field is never applied on add. The workaround is to manually set empty_data and return the field's viewData. Here's the revised version of the example.

$updateOnlinePayment = function (FormEvent $event) {
    $form = $event->getForm()->getParent();
    $order = $form->getData();


    $form->add('paymentMethod', 'choice', array(
        'disabled' => !$order->isOnlinePaymentSupported(),

        // as of 2.3
        'auto_initialize' => false
        // if parent form data has default value, this field will be empty if this option is not set.
        // datatransformer will also take effect
        'empty_data' => function($form) {
            return $form->getViewData();
        },
    ));
}

$builder->get('country')->addEventListener(FormEvents::POST_SET_DATA, $updateOnlinePayment);
$builder->get('country')->addEventListener(FormEvents::POST_SUBMIT, $updateOnlinePayment);

This may be achievable by split the form tree update and the deserialization (array -> object graph) as suggested in #6399 and others.

For me this is more usefull and intuitive than #5807.

I am closing this old issue as it didn't get any traction in years and it doesn't seem to be implemented in the future.