Thanks Jan! That was very informative and inline with what I was thinking.
One bit of feedback: Apple says that file coordination via NSFilePresenter and the like is not safe to use between processes. (See: https://developer.apple.com/library/ios/technotes/tn2408/_index.html). That's because NSFilePresenter isn't (yet) capable of dealing with process suspensions. If the app working with a file gets jetsamed, hits the memory threshold, or crashes, it will leave the lock on the file and other apps will deadlock trying to access it. Instead, Apple recommends using atomic saves for flat files.