Sorry, I do not. WordPress is like this too with no provision AFAIK for a non-standard port.
But, I was looking at bb-includes/db.php just now and the mysql_connect function is being used. That function can user
dbservername.com:port by default according to this page: http://www.php.net/function.mysql-connect
So, in theory, this should work:
Did you try that and it does not work?