simple.. add another field to the w3t_Boards table called showboard and set it to default to 0 and fopr the board you want to show set it to 1 and then in the categories.php and ubbthreads.php where it calls the board array do a
AND showboard = 0
then it won't show the forum you set to 1.
then add a link such as
http://www.yourdomain.com/ubbthreads/postlist.php/Cat/0/Board/keywordnameofboardand it will call the hidden board..