This is the code here: EmbeddedController/board/hx20/keyboard_customization.c at 553827caae7134d45a0617af9c201e333eab9a26 · FrameworkComputer/EmbeddedController · GitHub
It works by checking if Fn is pressed and then calling a different function for HID stuff. I think all you have to do is move the F10 and the rfkill/airplane mode would move with it but I could be wrong. There is only the hid enum and no scancode listed so I am pretty sure.