Remapping the `fn` key
Fn keys generate ACPI events and not keyboard events. To echo the keypress for Fn to the terminal use ...
sudo acpi_listen
Pres Fn + F12 and it will show
button/volumeup VOLUP 00000080 00000000 K.
This connects directly to the hardware and is OS independent. Remapping is out of the question.
Here is an interesting read about kernel and a keymap. But that goes beyond a basic changing of keycodes and it does not deal with setting the control key to act as an Fn.