Re: How to filter posts by Forum AND Tags?
The key is to edit the topic template so tags are shown as the ?tag=example&forum=123 (where 123 is the actual forum number)
Then you have to make a plugin that hooks
$topics = get_tagged_topics($tag->tag_id, $page);
do_action( 'bb_tag-single.php', $tag->tag_id );
where the function tag_forum_filter would manipulate (or regenerate) $topics based on whatever forum number $_GET is set to.