Hmm... it seems like what you're looking for is really an event. You might need to make some of your own Workflow-ng integration so that it invokes an event when a notification is received and passes as arguments the notification itself and the order affected. Then you'd need to have a condition that lets someone check what sort of message was received.

If this is too much trouble, updating the order in the code isn't always bad. You can always make it an option on the settings form. If you just want to "hard code" it, use uc_order_state_default() to use the default canceled status instead of setting the status directly.