Sounds like the best option. Haven't tried this so just writing queries from what I remember of them :-) :
admin/classes/order.php
around line 30:
function query($order_id) {
global $languages_id;
$order_id = tep_db_prepare_input($order_id);
$order_query = tep_db_query("select customers_id, customers_name, customers_company, customers_street_address, customers_suburb, customers_city, customers_postcode, customers_state, customers_country, customers_telephone, customers_email_address, customers_address_format_id, delivery_name, delivery_company, delivery_street_address, delivery_suburb, delivery_city, delivery_postcode, delivery_state, delivery_country, delivery_address_format_id, billing_name, billing_company, billing_street_address, billing_suburb, billing_city, billing_postcode, billing_state, billing_country, billing_address_format_id, payment_method, cc_type, cc_owner, cc_number, cc_expires, currency, currency_value, date_purchased, orders_status, last_modified from " . TABLE_ORDERS . " where orders_id = '" . (int)$order_id . "'");
$order = tep_db_fetch_array($order_query);
// BOF SPPC
$customer_group_query = tep_db_query("select cg.customers_group_name from " . TABLE_CUSTOMERS . " c, " . TABLE_CUSTOMERS_GROUPS . " cg on c.customers_group_id = cg. customers_group_id where c.customers_id = " . $order['customers_id'] . " ");
$customer_group_info = tep_db_fetch_array($customer_group_query);
// EOF SPPC
and then later (around line 71 now):
$this->customer = array('id' => $order['customers_id'],
'name' => $order['customers_name'],
'company' => $order['customers_company'],
'street_address' => $order['customers_street_address'],
'suburb' => $order['customers_suburb'],
'city' => $order['customers_city'],
'postcode' => $order['customers_postcode'],
'state' => $order['customers_state'],
'country' => array('title' => $order['customers_country']),
'format_id' => $order['customers_address_format_id'],
// BOF SPPC
'customers_group_name' => $customer_group_info['customers_group_name'],
// EOF SPPC
'telephone' => $order['customers_telephone'],
'email_address' => $order['customers_email_address']);