Dealing with macos Versions feature and file syncing services
Good news. After some prompting over in the Apple support forums and experimentation, I've discovered that the "close and revert" option does not do what it says. In fact it closes the document without touching the file on disk - exactly what I'm after!

To recap, when closing a modified file that supports Versions, this dialog appears:



There does not appear to be an option to close without changing what's on disk. However, the Revert Changes button actually behaves like a Don't Save button, leaving the file on disk untouched.

This is precisely the behaviour I need to support the case where the file on disk has been modified by another application and I want to preserve those changes.

As made clear in the Apple thread, this is all probably subject to change and may be fragile, but for now I have a solution.