You can promote based on posts, add a secondary group that provides elevated posting permissions on the nodes you wish. It allows you to easily expand and control what you want and is how the permission systems were designed.
Every member of your site should be a member of registered as their primary (including staff) and you add secondary user groups to elevate permissions. So if your moderators have no elevated permissions besides forum moderation permissions, you only set the permissions under Forum Moderator Permissions and nothing else. Then your moderators have the Primary Group of Registered and a secondary group of Moderating. The same applies to this type of situation, where your secondary group perovides only the elevated permissions you want for specific nodes (and nowhere else).