Is there a way for OSX to auto-copy a file somewhere whenever it's updated?
You would use something called Folder Action.

Right click on the Folder you want to monitor and select Folder action set up.

In the new window choose folder action workflow, and attach it.

Now select it and choose edit workflow, which will open Automator.

Here you do as shown below, while you can choose when it happens and where to save a copy.