Keyboard key to activate a Mesa output

I'm near complete converting another manual lathe (Standard Modern this time) to LinuxCNC.

These programmable keypads are convenient for commonly used key entries to Axis/MDI. From LinuxCNCs perspective they are just another keyboard.

I have a few spare keys and I would like to use one to activate a Mesa output. I can program the key to any key or even key sequence.

Is there a way to make this happen in HAL?

IIRC there is a file somewhere that either defines or lets you redefine key codes, I'm not sure if that will come into play? I suppose choosing an oddball key for this might be advantageous, so a typing a key on the regular keyboard does not inadvertently activate the Mesa output.