Modifying the X-Drive would be quite a challenge; it varies torque proportion to the front wheels continuously under dynamic conditions using inputs from speed sensors, yaw, steering angle, DSC, etc. Power to the servo motor in the transfer case is routed through the 70006 module itself. The programming and interpretation of the driving dynamics are complex, BUT the mechanism for the resulting proportional torque control at the transfer case itself is super simple. All you have there is a servo motor applying torque to a clutch pack through a lever.

If you wanted to control the torque proportion manually (100% rear, 50/50, whatever), you could easily do so by providing your own servo signal at the transfer case. Pin 1 is the servo control signal, Pin 8 is the ground ckt for the servo motor. Get the signal correct (it's not going to be anything special; servo motor control is nothing new or special, even in a BMW), and you can do whatever you want.

However, because of the intimate integration of the XDrive with the DSC, anti-lock braking, and related functions, I strongly suspect (actually, I am fairly certain) that any intervention will also disable those modules (thus the dash lights up like a Christmas tree, and those functions are all compromised). That's probably why you won't likely find any commercially produced software level over-rides to the programming. Nobody will want to be associated with changes that may compromise the stability system; even if you didn't sue them in the event of an accident, nothing says a third party wouldn't.

Not saying you shouldn't check it out and learn about the system if you want to, etc., but I doubt you'll find anyone providing a product to influence this area of the car.