How to order posts by the user_login that corresponds to a meta_value (which is a user id)?

Ok, solved this. I had to use the posts_clausesfilter and the following code:

function assigned_to_orderby( $clauses, $wp_query ) {
    global $wpdb;

    $clauses['join'] .= "
    LEFT OUTER JOIN {$wpdb->postmeta} ON {$wpdb->posts}.ID={$wpdb->postmeta}.post_id
    LEFT OUTER JOIN {$wpdb->users} ON {$wpdb->users}.ID={$wpdb->postmeta}.meta_value
    ";

    $clauses['where'] .= " AND ({$wpdb->postmeta}.meta_key = 'assignee')";
    $clauses['groupby'] = "{$wpdb->postmeta}.post_id";
    $clauses['orderby']  = "GROUP_CONCAT({$wpdb->users}.user_login ORDER BY {$wpdb->users}.user_login ASC) ";
    $clauses['orderby'] .= ( 'DESC' == strtoupper( $_GET['order'] ) ) ? 'DESC' : 'ASC';

    return $clauses;
}

Adapted from http://scribu.net/wordpress/sortable-taxonomy-columns.html