Using custom XML attributes for <block> nodes in Magento layout files

custom_xml_block_attributes_block.php

<?php
// This code must in a block class, child of Mage_Core_Block_Abstract
// Retrieve value of 'alt-template' attribute from layout for current block
$blockElement = $this->getLayout()->getXpath("*/block[@name='" . $this->getNameInLayout() . "']");
if($blockElement) {
    /* @var $blockElement Mage_Core_Model_Layout_Element */
    $blockElement = end($blockElement);
    $altTemplate = $blockElement->getAttribute('alt-template');
    if(!is_null($altTemplate)) {
        $this->setTemplate($altTemplate); // Or your logic...
    }
}
?>

custom_xml_block_attributes_layout.xml

<!-- Example using alt-template attribute -->
<block
  type="core/template"
  name="my.block"
  template="path/to/regular/template.phtml"
  alt-template="path/to/alt/template.phtml"/>