Try the xev command and press Fn.
On my computer (Lenovo T60), I see this in the terminal when I press the Fn key:
KeyPress event, serial 33, synthetic NO, window 0x3c00001, root 0x6d, subw 0x0, time 2266123481, (652,782), root:(655,915), state 0x10, keycode 151 (keysym 0x1008ff2b, XF86WakeUp),
So, the Fn key is key code 151. My left control is 37. Then you can reassign, knowing which key codes they generate.
https://web.archive.org/web/20090202093048/http://www.columbia.edu/~djv/docs/keyremap.html
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