If anyone is interested I've created a Google Customer Review Opt-in checkout success module.
In particular very helpful to add customer ratings to your Google shopping ads.
1) Google does not display the review opt-in to every customer
2) You need more than 100 independent Google reviews before G will even show the Opt-in
3) A Google Merchant Center ID is required
Support thread here: https://forums.oscommerce.com/topic/493682-google-customer-review-optin-checkout-success-module/
Should work on any version of CE version....