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Today I tried remapping the Fn-key on my keyboard after my Ctrl-key stopped working reliably (I suspect it's a hardware issue). While I first suspected this to not be possible at all, because a lot of keyboards do not even send the Keypress Events from the Fn-keys to the kernel, for my laptop keyboard this seems to have not been the case, as I was able to see the keypresses with "xev". Next, I tried rebinding the key with xmodmap using its keycode (151): xmodmap -e 'keycode 151 = Control_L', but that did not work for mysterious reasons.
After some more failed attempts, I tried using setkeycodes, which directly changes the map of the keyboard-driver to change which keycode would be returned upon pressing the scan-codes sent by the keyboard. Remapping the Fn key there did work, but for to me mysterious reasons, the keycodes for setkeycodes are offset by 8, so I ran "sudo setkeycodes e063 29" which successfully bound the key to keycode 37 according to xev when I ran it afterward. Apparently the keycodes sent by kernel are different than the ones by xev.
Afterwards I realized I could have looked for the corresponding archwiki entry about keyboard input long ago to get an overview of how this roughly works on linux. It mentions complications like the offset between kernel and Xorg keycodes.