There is a hacky workaround to identify the base product region using the “HTML API” (I presented it in a lighting talk in the latest App Week).
Make anonymous request to https://{site}.atlassian.net/plugins/servlet/ac/foo/bar and in the HTML response find <meta name="ajs-region" content="..."> which contains the region where the base product is located (e.g. prod-west2 or prod-euwest or similar).
BTW we are also thinking about a similar solution to have some computing and data storage resources centralized and some decentralized to several data residency regions and having a single baseURL in the descriptor. But in our case we plan to provide customers the option to migrate their data to selected data residency region instead of automatically detecting it.
Kind regards,
Raimonds