Ok, just finished some testing to get a brushless ESC working with the Spit's AS3X brick. Changing the brick into brushless mode in the same way as the AR6400 I have already established through testing and confirmation from Horizon does not work and cannot be made to work.
However I have a solution that does work.
Using the programming information for the AR6400 I first changed the second reversed aileron servo socket on the brick back to be channel 6. Then connected the servo input leads from the external ESC to the now channel 6 servo socket on the Spit's brick. I mixed channel 6 on my transmitter to throttle on my transmitter and I now have brushless power that works for the Spit's AS3X brick.
Now I can try and shoehorn the Spit's AS3X brick into my FW190