AccountPage_ControllerExtension.php

<?php

/**
 * Class AccountPage_Controller
 *
 * @property AccountPage_Controller $owner
 */
class AccountPage_ControllerExtension extends Extension {

    private static $allowed_actions = ['pastorders'];

    public function pastorders() {
        return [
            'ShowPastOrders' => true
        ];
    }

    public static function add_to_class($class, $extensionClass, $args = null) {
        //this does get called, and $class === AccountPage_Controller
    }
}