How can I access or expose the Commerce data in $order->data[] to the Rules module?

Following this old checkout_pane_example.module tutorial, I created a custom Drupal 7 Commerce checkout pane. It's simply a text field to collect special order instructions from the customer.

function checkout_pane_example_pane_checkout_form($form, &$form_state, $checkout_pane, $order) {

  $pane_form['nickname'] = array(
    '#type' => 'textfield', 
    '#title' => t('Your nickname'), 
    '#description' => t("We'd like to call you by your nickname."), 
    '#default_value' => !empty($order->data['nickname']) ? $order->data['nickname'] : '', 
    '#required' => TRUE,
  );

  return $pane_form;
}

The pane works fine. Anything that's saved in the text area is then stored in $order->data['nickname'].

The problem is that I need to access the data in Rules so I can attach it to the existing email that's sent out on order completion email.

How can I access or expose that data via the Rules module?

You can't really expose data as such, but you can define your own order property via hook_entity_property_info() with a custom callback that returns the specific value from the data array you want.

For an example, take a look at commerce_order_entity_property_info()'s definition of the mail_username property and the related commerce_order_get_properties() callback.