Orderby meta_value last_name via WP_User_Query Object

user_query.php

<?php

// Caveat: This creates 2 queries and any use of this code should also use site transients

// prepare arguments
$args  = array(
	'meta_key' => 'last_name',
);

// Create the WP_User_Query object
$author_query = new WP_User_Query( $args );

// Modify the WP_User_Query object
$author_query->query_orderby = str_replace( 'user_login', 'wpsand_usermeta.meta_value', $author_query->query_orderby );

// Re-query
$author_query->query();