Exposing this somehow via the REST interface would also be useful, potentially much more useful than simply reporting some stats within the application. For my use-case, a way to pull lists of users inactive for N days (or whatever time resolution seems practical to developers) would be extremely useful, so that I can then flag those users, notify them they’re pending deletion, or whatever.
While I’m on it… a way to identify channels that haven’t had recent activity could also be useful, but I’m betting that’s an entirely separate can of worms (and a different feature request).