Update on this... a new feature is now available that automatically updates the num_available field in your database based on changes to users' Orders during the current order cycle.
So if you set num_available to say 5 for an item, then only a total qty of 5 of that item can be ordered by all users. It will correctly reduce num_available as people add the item to their Order, and then increase it again if they delete the item from their Order. However, it does not support Splits yet. Filled splits will not affect an item's num_available.
One more caveat: it only works if phpMyAdmin is enabled to maintain your private database, and for regional databases which are designated as having an accurate num_available value.