Have you considered enabling cookies auth which has timeout of 0 (i.e. session)

That wouldn't be the PHP sessions as such,
but would have some of same benefits.

As it is now, it's not AFAIK possible as you calc:
$cookieexptime + (time() + $cookie_expires)
[thus doesn't help setting $cookie_expires = 0]

So, just edit bb_cookie.php and set $cookieexptime=0; - what's so difficult? :-)