John,
I have since spoken to one of the manufacturers, Fogtec. They have explained the concept of there system in this application. The system comprises a water cylinder and a N2 cylinder. The N2 cylinder pressurises the water cylinder only and leaves the pipe work at the end of the discharge. The system has been tested by a wind turbine manufacturer and works. It is an extinguishing system rather than a suppression system, so if the fire has burnt through the cabin/housing and is effecting the blades then it has not done its job. You are spot on about water mist being ineffectual outside as it is blown everywhere except where you want it, but in an enclosed space it works very well. This is especially so if the fire is very hot as the water mist droplets turn to steam very quickly!