I had a similar situation a while back. I was flying on an MU codeshare operated by FM (back then, still a *A member) while I held UA*G status. I emailed them prior to flying asking if I would get *G benefits or miles and they replied no. I found this strange that not even benefits would be given since I could simply show my UA*G card for lounge, boarding etc. and I sent a message to Star Alliance Services GmbH. The Director of Customer Relations there replied saying that as long as the operating carrier is a *A member, I will get *G benefits and miles (as long as the MU codeshare maps into an eligible booking class with FM) and apologized that FM was wrong.