I need a hack like this one too, and i have an idea how to make(it doesnt workd with PASSSWORD protected frum,but with them who are restricted to special members...)
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When a user logs in, you could check if he is allowed to view the private forum...If he is, show it...if not,dont show it...
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Hope u understood that(my english is really bad) and someone could a code a "hide private forums"hack...That would be veryververy cool!
PS:i dont like the idea with the second ultimatebb.cgi,because its not as comortable like my idea above...