Override Mage_Adminhtml_Block_Sales_Items_Column_Name Block

Question

I'm trying to add an extra data to the file:

/app/design/adminhtml/default/ad02/template/downloadable/sales/items/column/downloadable/name.phtml

Target idea: provide a link with a href="....":

<?php if ($this->getLinks()): ?>
        <dl class="item-options">
            <dt><?php echo $this->getLinksTitle(); ?></dt>
            <?php foreach ($this->getLinks()->getPurchasedItems() as $_link): ?>
                <dd>
                  <a title="Download" href="<?php echo $_link->getLinkHref() ?>">
                    <?php echo $_link->getLinkTitle() ?>
                  </a> (<?php echo $_link->getNumberOfDownloadsUsed() . ' / ' . ($_link->getNumberOfDownloadsBought()?$_link->getNumberOfDownloadsBought():Mage::helper('downloadable')->__('U')) ?>)</dd>
            <?php endforeach; ?>
        </dl>
    <?php endif; ?>
    <?php echo $this->htmlEscape($_item->getDescription()) ?>

I suppose to do that by extending a related Magento Block with a new method getLinkHref()(see above). This is current Block Class contents:

class Mage_Downloadable_Block_Adminhtml_Sales_Items_Column_Downloadable_Name extends Mage_Adminhtml_Block_Sales_Items_Column_Name
{
    protected $_purchased = null;
    public function getLinks()
    {
        $this->_purchased = Mage::getModel('downloadable/link_purchased')
            ->load($this->getItem()->getOrder()->getId(), 'order_id');
        $purchasedItem = Mage::getModel('downloadable/link_purchased_item')->getCollection()
            ->addFieldToFilter('order_item_id', $this->getItem()->getId());
        $this->_purchased->setPurchasedItems($purchasedItem);
        return $this->_purchased;
    }

    public function getLinksTitle()
    {
        if ($this->_purchased && $this->_purchased->getLinkSectionTitle()) {
            return $this->_purchased->getLinkSectionTitle();
        }
        return Mage::getStoreConfig(Mage_Downloadable_Model_Link::XML_PATH_LINKS_TITLE);
    }

    /* I want to insert this method */
    public function getLinksHref()
    {
        return 'http://google.com/'
    }
}

But. I don't want to edit the original file but I want to override this Block Class. And actually the question is how to Override this block?

My Module's config.xml

<?xml version="1.0"?> <config>
    <modules>
        <AD_DownloadablesGeneratedLink>
            <version>1.0.0</version>
        </AD_DownloadablesGeneratedLink>
    </modules>
    <global>
        <blocks>
            <adminhtml>
                <rewrite>
                    <order_items>AD_DownloadablesGeneratedLink_Block_Adminhtml_Sales_Items_Column_Downloadable_Name</order_items>
                </rewrite>
            </adminhtml>
            <AD_DownloadablesGeneratedLink>
                <class>AD_DownloadablesGeneratedLink_Block</class>
            </AD_DownloadablesGeneratedLink>
        </blocks>
     </global>

Thank you so much for you help in advance!

P.S. v1.4.0.1