bbPress.org

P.S. It is possible to achive the output I want by hacking the common/widgets.php file, (not that I am suggesting this is good, as will be overwritten on upgrades!)
in widgets.php, changing the Topics widget query on lines 743 and 764 as below does what I want! Maybe not too hard for author to add customise this properly?
HACK:

			// Order by most recent replies
			case 'freshness' :
				$topics_query = array(
					'post_type'      => array(bbp_get_topic_post_type(),bbp_get_reply_post_type() ), //HACKED!
					// 'post_type'      => bbp_get_topic_post_type(),