Auto sort the wp-admin post list by a meta key

Something like this should work:

function wpa84258_admin_posts_sort_last_name( $query ){
    global $pagenow;
    if( is_admin()
        && 'edit.php' == $pagenow
        && !isset( $_GET['orderby'] )
        && !isset( $_GET['post_type'] ) ){
            $query->set( 'meta_key', 'last_name' );
            $query->set( 'orderby', 'meta_value' );
            $query->set( 'order', 'ASC' );
    }
}
add_action( 'pre_get_posts', 'wpa84258_admin_posts_sort_last_name' );