Thanks for posting your question. Hopefully you saw Leslie Martensen's response to this topic in the Contract Modification Reporting discussion thread. As Leslie noted, this feature is currently not available as an out-of-the-box offering and its something that she's exploring with customers. In that same discussion thread, Scott Corbett indicated that he's doing something similiar - you may want to reach out to Scott to learn more about what's he's doing.
Ariba Exchange Community Manager