Right, as far as I can tell, getting assignment group grades (or the Final) grade does not seem possible via the API, and to request it, you'd need to propose an idea as mentioned earlier, or wait until Instructure announces the new feature request system.
But by following the network requests when downloading the gradebook CSV, it might be possible to replicate in a script.
It first sends a POST request to /courses/[course_ID]/gradebook_csv with a JSON listing the assignment IDs, the students (both of which would likely need to be extracted using the API) and some options.
Then it uses the API to get the progress of the CSV creation: /api/v1/progress/[fileID]
Then the API again to get the CSV download path: /api/v1/users/[userID]files/[anotherFileID]
Then it uses the link got in the last step to download the file.
Would be easier to do with the API, but might still be feasible.