Get all tags used in custom post type

I faced the same problem.
Originally, I was going to solve it with this:
http://wordpress.org/support/topic/custom-post-types-and-tag-cloud?replies=8
but that solution seems a bit inefficient because of the need to get each and every post’s tags and then to remove duplicates.

I ended up doing this:

function custom_posts_per_tag($id, $post_type){

	$args = array(
        'post_type' => array($post_type),
        'posts_per_page' => -1,
				'tag_id' => $id
  );

	$the_query = new WP_Query( $args );
	wp_reset_query();

	return sizeof($the_query->posts);
}

$tags = get_tags();
global $wp_query;
$post_type = $wp_query->get('post_type');

foreach ($tags as $tag){
	if (custom_posts_per_tag($tag->term_id, $post_type) > 0) {
		/* Print tag link or whatever you wish here */
	}
}

I assume that WP_Query uses a more efficient way to filter posts by tags, than getting each post’s tag, but I might be wrong.