You can remap the capslock key to escape. Or do what I do, have capslock be ctrl when combined with a non modifier key, and escape otherwise. I enter normal mode with my pinky.
Re-read your comment, how do you do this at OS level? I can imagine doing this in Vim or Tmux, but OS level for any application?
b.t.w on Happy Hacking keyboard that button is CONTROL by default.