I am able to get all Multiregion and their respective zones from https://cloud.google.com/about/locations/
As shhown in pictures, right now there are 3 Multiregion available in GCP.
Is there any API/JSON available for finding the relationship between MultiRegion - Region and Zone in GCP?
Finally I have created the Feature Request.
You will have to rely for a while on your solution since any kind of the API implementation follows strict rules and rollout procedures.
You can follow the updates in the public issue tracker, I suggest you to star it to receives updates and notifications and place a comment saying why it would be useful in your case. The speed of the implementation depends as well on the attention raised.