One little trick I just discovered is that you can create an admin with no admin permissions:
Admin CP -> Users -> Administrator -> Create New Administrator
An admin without permissions has the is_admin flag set which gets them into the forum while it's closed. They can login to the Admin CP but they have no access except for:
Admin CP -> Tools -> Rebuild Caches
If you don't like that solution then you can dig into the code. This is the relevant code from what I can see:
library/XenForo/ControllerPublic/Abstract.php
Code:
protected function _assertBoardActive($action)
{
$options = XenForo_Application::get('options');
if (!$options->boardActive && !XenForo_Visitor::getInstance()->get('is_admin'))
{
throw new XenForo_Exception($options->boardInactiveMessage, true);
}
}
Don't forget you can also add .htaccess to the admin.php file and not give the login details to your mods that shouldn't have access.
And if you have a few admins but have an admin only forum, make sure the node permissions for usergroup moderators is set to DENY on the admin forum instead of inherit, that way they don't get access to the admin section