Wow, glad you're ok.
Doesn't BMW have a failsafe on their brake system? In theory every car should have this. A cross link between RF and LR, or maybe front and back. This type of system is designed specifically for these applications - when you have a puncture and all fluid doesn't gush out at the same time. I know Volvo cars have that.
Also, after you started sliding, did you notice any "Brake Fluid" notification on the dash?
I had my brakes fail on me at 140km/h in a 2005 Volvo V50. Same situation, pedal to the floor. I was able to get very little braking power by pumping the brakes. Eventually came to a safe stop. The reason for failure was a punctured (non-stainless steel line) brake line. After applying the brake, I had brief stopping power, and then a solid thumb of brakes letting go and pedal down on the floor. The brake warning came on ~15 seconds after I lost my brakes. So much for the safest car in the world.
Good to hear you're ok. Can you post some pictures of where the puncture happened? What type of calipers did you use?