It looks like basically you need to be able to tell if the DragManager craps out. Even if there was an ERROR event in the drag/drop system (there isn't that I've seen, but maybe I've missed something obvious), you probably couldn't always count on it.
Sort of like trying to set up a client to know when the server has died kind of thing, yeah?
I may be way off, but I'm thinking you could add a mouseMove listener when you begin your drag and constantly check to see if you are still dragging. I believe you can dynamically set how often that mouseMove event fires.
And, of course, once you complete the drag - whether maturely or prematurely - remove the mouseMove listener.
I'm really not crazy about doing something like this. Adding a babysitter to an operation. But I think it'd do what needs doing, and I don't think it needs to be messy.