Yep - I love my custom mapping questions 😎!
I've mapped the FX3 knob (4th knob) on Decks A & B to the mixer Key Adjust for each channel. I use it for a smooth key effect for buildups.
Is it possible to map the FX3 button to reset the Key adjust to the center line? That way when I sweep the key for the buildup, I press the button to reset it rather than have to sweep they key effect down, which is sometimes audible in a way that I don't like.