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The Fn key is not sent to the OS at all, this would not be remappable via OS mechanisms.

That depends on the laptop. On my Thinkpad X220, pressing the Fn key triggers a normal KeyPress event with keycode 151, so no problem remapping.

On my T41 too, with the same keycode, but not on my old Samsung. Considering I almost exclusively use ThinkPads, I just left Fn where it is.