Okay, first, so you know, I don’t know anything about reverse geocoding, but it sounds like you do, so I’m hoping that my ignorance in this area doesn’t matter.
Does this reverse geocoding need to be done with JavaScript or can it be done in PHP. I assume you’re connecting to the google API to do it? If this is possible you can create an acf/save_post action, http://www.advancedcustomfields.com/resources/acfsave_post/ and in that function get the saved map field, do the api call with PHP and then save the result into another field.
Hopefully this is some help.
~JH
