FN keys are usually handled at a firmware, sometimes BIOS or driver level. This makes them completely inaccessible to keyboard remappers (they have no idea when it is pressed), which is why none of them can do this.
This Stackexchange answer looks like it should contain the information you need, but I don't have a 2014 MacBook to test it.
You can also use
<C-C>to quit command mode instead (and this one works in keymaps). It also works in Insert mode but doesn't trigger theInsertLeaveevent, which might upset some plugins.As far as I know the character pending mode thing that
rdoes is a bit unique in that it isn't insert mode, but also not quite replace or operator pending mode. If there's a way to remap any key to work withrthen I'm not aware of it.Have you considered instead remapping
<Esc>on an OS level? The XKB optioncaps:escapeturns your Caps Lock key into an additional Escape key, which is even easier to type than<C-Space>.