Sort Magento Collection by Attribute Option Value?

Question

I have rewritten Mage_Catalog_Block_Product_List:

$this->_productCollection = $layer->getProductCollection();
$this->_productCollection->addAttributeToSort('product_type', 'ASC', true);

Where the product_type attribute has an option value=>key:

<option value="515">Mount</option>
<option value="624" selected="selected">Accessories</option>

I want my product collection to sort by option key and not by option value text.

Is this possible with AttributeToSort?