Taking this one step further, if the internal wastegate is controlled by an interupt as per your diagram, couldn't you set the max duty at 100 with a target boost that was really high at 100% throttle? That should create the following scenario:
Target boost at 18psi and WGDC normal in 85% throttle column.
Target boost at 21 psi and WGDC at 100% in 86% throttle and up column.
MBC at 19psi.
Boost below 19 psi would be controlled by the internal wastegate only.
At above 85% throttle, the target boost will jump up and drive the WGDC to 100% closing the internal gate. Boost is now controlled by the external gate only.
I haven't tested this as I don't have an external gate yet, but the theory seems sound. You could set the crossover point for whatever boost and throttle % you want. Personally I would change the 100% throttle column to a value close to the crossover (like 86% above.) That way you would have a clear switchover point and then adjust it as needed for a smooth transition.