I think it is a good idea NOT to model all the backward transitions. Unfortunetely there is no out-of-the-box way of undoing (means rolling back a business transaction) at the moment. And some things have to be considered, like compensating actions and the like.
There was a discussion about it already a couople of times, but if I am right, it is not yet built in. We always implemented it project specific if required.
How hard was implementing it project-specific? Is that code reusable? :)