Add new variable to order update e-mail

possible solution:

You need to override the Order model (app/code/core/Mage/Sales/Model/Order.php) and for the sendOrderUpdateEmail() method
you need to change this section

$mailTemplate->setDesignConfig(array('area'=>'frontend', 'store' => $this->getStoreId()))
->sendTransactional(
$template,
Mage::getStoreConfig(self::XML_PATH_UPDATE_EMAIL_IDENTITY, $this->getStoreId()),
$recipient['email'],
$recipient['name'],
array(
'order'     => $this,
'billing'   => $this->getBillingAddress(),
'comment'   => $comment
)
);

to this:

$mailTemplate->setDesignConfig(array('area'=>'frontend', 'store' => $this->getStoreId()))
->sendTransactional(
$template,
Mage::getStoreConfig(self::XML_PATH_UPDATE_EMAIL_IDENTITY, $this->getStoreId()),
$recipient['email'],
$recipient['name'],
array(
'order'     => $this,
'billing'   => $this->getBillingAddress(),
'comment'   => $comment,
'your_var_name'=>'SOMETHNG'
)
);

In the e-mail template you can show the new var like this

{{var your_var_name}}