I've found that the Fn key is not accepted as a modifier hotkey for a trigger, is there a reason for this?
I guess the fn key is too low level. You can’t use it as modifier for macOS shortcuts either.
I don't know the reason, but if you want to change this, I recommend Karabiner.
I use Karabiner for the caps lock key but not sure I want to change the Fn key's functionality. In the case of the caps lock I did.
Generally, the Fn key is not a real key, not a modifier like Option or Shift, but instead is a hardware toggle, that toggles the meaning of the function keys between their function key behaviour and their hardware key behaviour.
The Fn key is not supported by the system Hot Key API.
However, if you're using a non-Apple keyboard (I'm typing this on a Microsoft Natural Ergonomic 4000 keyboard) you might have more luck. I'm using the F-Lock key as a toggle for my microphone.