Get notified when an item runs out of stock in MWS

If I'm listing multiple items with different quantities, is there a way to get notified when I run out of stock for any of them? Of course if there is a report that can return the currently out of stock items then that would be great too.

I'm using the MFN (Merchant Fulfillment Network) and there are millions of items currently listed on Amazon.

What I have tried:

1. The Reports API has the inventory report which returns all stock, but it can be requested only once per day (since we have a catalogue of millions of books) and it takes about 5 hours to be generated.
2. The fulfilment API has a function ListInventorySupply, but it works only for AFN (Amazon fulfilment network) so it work only for fulfilment by Amazon sellers.