[_ck_]

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 );

add_action(‘bb_tag-single.php’,’tag_forum_filter’)

where the function tag_forum_filter would manipulate (or regenerate) $topics based on whatever forum number $_GET is set to.