In reply to this topic : http://bp-fr.net/groupes/utilisation-configuration-optimisation-de-buddypress/forum/topic/comment-ajouter-un-nouveau-critere-de-tri-aux-topic/ . Adding a new nav item to BuddyPress Forum dir (in case of bbPress 1.2 powered forums..) for a given topic tag.

bp-tag-nav-item.php

<?php
/*** Beginning of the code to paste into the functions.php of your BuddyPress theme / child theme ***/

/**
 * Hooks bp_forums_directory_group_types and adds a forum directory nav item
 *
 * @global $wp_taxonomy_object
 * @uses $wp_taxonomy_object->get_term_by() to fetch tag datas by name
 * @uses bb_get_tag_link() to get permalink for the given tag id.
 * @author imath
 */
function jose_adds_forum_tag_nav() {
	global $wp_taxonomy_object;
	
	/* the name of your bbPress (1.2 for BuddyPress group forums) tag */
	$tag = 'support';
	
	/* get tag data */
	$bb_term_object = $wp_taxonomy_object->get_term_by( 'name', $tag, 'bb_topic_tag' );
	
	/* number of topics that uses this tag */
	$count = $bb_term_object->count;
	
	/* permalink to tag */
	$tag_link = bb_get_tag_link( intval( $bb_term_object->term_id ) );
	
	?>
	<li id="forum-dir-tag" class="no-ajax"><a href="<?php echo trailingslashit( $tag_link ); ?>" data-tagname="support">support <span class="tag-count"><?php echo $count;?></span></a></li>
	<?php
}

add_action( 'bp_forums_directory_group_types', 'jose_adds_forum_tag_nav' );

/**
 * Hooks bp_after_directory_forums_page to inject some javascript
 *
 * In bp-default/_inc/ global.js launch ajax actions that are catched by ajax.php
 * In previous jose_adds_forum_tag_nav we added a no-ajax class to li#forum-dir-tag & in order
 * to avoid global.js to run its script, the following script will use global.js bp_ajax_request
 * to load the forum template for the given tag request.
 * @author imath
 */
function jose_custom_js() {
	?>
	<script type="text/javascript">
	/* <![CDATA[ */
	jQuery(document).ready(function($){
		
		$('.item-list-tabs ul li a').click( function(){
			if ( $(this).parent().attr('id') == 'forum-dir-tag' ) {
				var li = $(this).parent();
				var ulparent = li.parent();
				
				// let's remove any selected li
				ulparent.find('li').each( function(){
					if( $(this).hasClass('selected') )
						$(this).removeClass('selected');
				});
				
				//let's add our li as selected and loading..
				li.addClass('selected current');
				li.addClass('loading');
				
				// the tag to request
				var tagname = $(this).attr('data-tagname');
				
				if ( bp_ajax_request )
					bp_ajax_request.abort();

				bp_ajax_request = jq.post( ajaxurl, {
					action: 'forums_filter',
					'cookie': encodeURIComponent(document.cookie),
					'object': 'forums',
					'filter': 'tags',
					'search_terms': tagname,
					'scope': jq.cookie('bp-forums-scope'),
					'page': 1,
					'extras': jq.cookie('bp-forums-extras')
				},
				function(response)
				{
					jq('div.forums').fadeOut( 100, function() {
						jq(this).html(response);
						jq(this).fadeIn(100);
					});
					jq('.item-list-tabs li.selected').removeClass('loading');
				});

				return false;
			} else {
				$('#forum-dir-tag').removeClass('selected current');
			}
			
		});
		
	});
	/* ]]> */
	</script>
	<?php
}

add_action( 'bp_after_directory_forums_page', 'jose_custom_js' );

/*** end of the code to paste ***/
?>