[Logician]

I see some problems with the idea:
1- It would be problems regarding user notification: Say your board is open at 01.00 and someone replied a thread I have subscription at 02.00, what if I choose to follow the notification mail link at 18.00 (where board is hidden)? It would cause confusions.

2- It would be a real problem to hack searching options of vb. Someone searching a keyword which resides in a post in that forum will be able to access the message (at least some part of it) via search.php even if the board is closed.

If I were you I would apply group permissions to that forum and would allow approved members' access (in any hour) and close to board to everyone else. IMO this is the best solution.

But if you are determined to apply your own solution, I would recommend using such a method to make it work better:
Create a small script (in fact 2) and overwrite the forum permissions of that specific forum via these scripts. One should open the board to all and the other will make it private and open to only some usergroups (Admin, Mod). Then you can relate these 2 scripts to your server's cron job and they will auotmatically make your forum visible/invisible in the programmed hour.

I didnt check the hack mentioned, so dont know what kind of logic it's using but IMO this would be most solid algortihm for this hack..