In the old X5 (e53) if you had the nav you had the option to enter a code at turn off. When you returned to the car before it would start you had to input the code you had set on leaving the vehicle into the nav system before it would start and operate. Basically like what you're describing but it was integrated.
I don't see why BMW can't implement this feature in the newer iDrive models, it seems a bit of a step back in my opinion. I shouldn't have to have something fitted to my vehicle if an older model did it in the first place.