have to set allowposting on/off on delay

allowposting is a permission, so set in the bitfields...
( $fmanager->set_bitfield('options', 'allowposting', 1); )

i need to set all forum tagged X to be closed at X hour... i just need to know how to set the permission per forum, and i can't find it...

i have my cron jobs for both situation (open/close) but i need to know the best way to deal with these two settings:

1- select each forum that have the proper tag
2- for each of these forums, set the proper permission and save

can someone help on that stupid task ?!

... btw, this gadget would be released to let admins set a forum they want to be open/closed on certain hours per day.. (like a nightlife forum...)