The best way to test drive very expensive cars is to show up dressed nicely (a suit for instance) and driving an already expensive car.
For instance, I am sure I could test drive an M5 if I drove up in my Audi S4 (which is in pristine condition) and wearing a suit, and I bet I could do it without a deposit. I was not required to put a deposit on my S4 test drives (all 3 of them) and while I don't have much experience at BMW dealerships, I doubt they would balk.
If you can walk the walk, you'll get the keys
