@Paul: As of now, we don't use that Mac App Store Updates URL yet, but that's mainly because I had no idea it existed. It would definitely make sense to show that in cases where the running app version is older than the newest one shown in the change log.

Now to find out if the macappstore URI scheme is officially supported by Apple. And I wonder whether what Jiulong Zhao says about sandboxing on Stack Overflow is true — although that's irrelevant for most of our non-sandboxable apps.