Sorry for the delay. I always forget to subscribe to my plugin's forums.
You can order the WP_User_Query with the following orderby parameters:
I don't really know how you'd do it by a custom field. You might have to write your own where and orderby clauses. You can change my plugin's query args via filter
$args = apply_filters( 'sul_user_query_args', array_merge( $args1, $args2 ) );
So for example, you could order by post_count
It looks like you can query by meta fields:
But I couldn't tell you how to make them play together. Maybe you can try setting the orderby parameter to metakey. If you figure it out, let me know. Also, let me know if you'd like me to add some parameters to the shortcode.