wordpress-custom-orderby.php

<?php
/**
* instruct wordpress what orderby=template means
* @param Array wordpress request rules
* @return Array modified $vars
*/
function template_column_orderby( $vars ) {
    // the search extras that will order correctly
    if ( isset( $vars['orderby'] ) && 'template' == $vars['orderby'] ) {
        $vars = array_merge( $vars, array( // merge the existing search parameters with ours
            'meta_key' => '_wp_page_template', // this is the meta vale to order by
            'orderby' => 'meta_value title', // order by the meta_key we define, then by title
            /* 
            * ordering the pages by a meta key, will only return pages with a value present.
            * to get by this, we tell Wordpress to include pages with or without the value
            */
            'meta_query' => array(
                'relation' => 'OR',
                array(
                    'key' => '_wp_page_template',
                    'compare' => 'NOT EXISTS'
                ),
                array(
                    'key' => '_wp_page_template',
                    'compare' => 'EXISTS'
                ),
            )
        ) );
    }
    return $vars;
}

add_filter('request', 'template_column_orderby');