In my experience, you need to start the push from an Authorized BMW dealer center. Calling/contact BMWUSA directly will yield to these type of responses (they cannot support companies not within BMW's standards). I understand their type of response - If I'm BMWUSA, and a customer contacted me to pay for an engine replacement with a shop that I have no affiliation with, nor do I know their standards of work, I wouldn't help out either.
Best chance is to have an Authorized dealership batting for you when talking with BMWUSA. I had a similar issue with my E55. I had a very reputable shop diagnose a transmission issue. I called up the dealership, and they said I need to bring it in. I called MBUSA, and they said they can't help me. I talked with the service manager at the local dealership, saying I'm a good customer. They called MBUSA, and they worked it out that they would pay for my diagnosis from the independent shop (tear apart of the trans), transportation of the vehicle to the dealership and payments of all parts.