I never have "Num Lk" on, as there isn't a separate keypad. Sony basically put a pseudo keypad in the middle of the keyboard which is a pain to use.
There is no response on "acpi_listen" when I press the "Fn" key with or without the "Num Lk" on.
I am not worried about the "Fn" key not working. The only two keys I ever use are the backlight controls and that has been fixed with the keyboard shortcuts.