Ya,
I guess when it does the transactions approved which puts the stock level to zero then you would trigger an event like "Stock depleted", for example. This would be at the API level as it interacts with the rest of Ubercart.

I might be revising that module as I intend to integrate with a third party stock tracking system so this will go in my notes. I want to integrate with the 'Acions' in Drupal 6 so it will be for UC2.