I just bought the Grip Daddy Bracket for my 550 and its perfect.
http://gripdaddymounts.com/v2arm.html
I use it all the time for my kids on road trips and its puts the IPad right in the center between the two front seats. If you decide to order make sure you tell them you need bigger hooks for the BMW. The hooks that come with it are too small for the thick posts on the headrest.