Sharing as well the feedback I received from Shopify support, which led me to believe that adding detection of region ID isn’t an option, even if we did get a data feed for it.
I definitely understand the frustration with this, but Shopify’s platform is not setup in such a way to allow modification to the URL structure of your products internally, especially based on inventory availability.
I wish I was able to provide further assistance on this at this time but our Marketplace team states that it’s not a feature currently built into the platform or the Google Shopping app, but have acknowledged that it is an inquiry they’ve been getting more and more.