The filter is well hidden, but you can track it down from tag_description ($tag), which calls term_description ($term=$tag, $taxonomy = 'post_tag'), which in turn calls get_term_field ('description', $tag, 'post_tag'). And finally you get the call sanitize_term_field ('description', $tag->$description, $tag->tag_id, $tag->post_tag, 'display').
Once you’re in sanitize_term_field you see a whole lot of filters. It looks like you need the last one, which would translate to tag_description, but given that I’m doing the parsing from the top of my head some debugging might be necessary.