How do we reproduce this in a plugin and deal with dependencies / priority so that it loads properly? I would also suggest the documentation (http://www.advancedcustomfields.com/resources/local-json/) be updated to reflect the strong movement towards moving functionality out of the theme.
The problem with setting the save point to somewhere in you plugin is that there can only be one save point. So if you override it your plugin then all saves will go to your plugin folder, unless that’s what you’re aiming for. However, as another developer that uses ACF I wouldn’t be to happy about a plugin changing the save point, just saying.