Escape can be mapped to whatever You want in hardware keyboard (without loosing current functions, see "remapping hardware keyboard" wiki article). It *may* not work, when mapped to layer 4 backspace, return or arrows, but - for example - layer 4 "s" (fn + shift + s) works flawlessly.

This way, I got F1-F11 mapped to layer 4 upper row keys, together with all (polish) special characters, + all special symbols found on "regular" computer keyboards. Without loosing any default symbol present on hardware keyboard - layer 4 is *not* used in vanilla keyboard mapping.So, we got plenty (almost all) keys for mapping.