[calorie]

Untested but there is $stylevar['textdirection'] on vB3 that you could set to rtl instead of ltr if in a specific forum. You might be able to accomplish this with a plugin at global_start using $forumid so it'd be something like this:

// change X to the forum ID number
if ($forumid == X)
{
	$stylevar['textdirection'] == 'rtl';
	$show['rtl'] = 1;
}