Sexy idea, but as someone who started using vi in 1987, a software solution seems way better. I map the seldom-otherwise-used key sequence jk to <ESC>. If I were doing it over, kj would be slightly better, for times when I am Just Kidding. I do this in both .vimrc and .inputrc (using different syntax, sigh). My fingers generate the escape without thought. (As is true for tons of vim normal mode actions I use regularly.)
I briefly considered a foot pedal but it seems to me that foot dexterity is far lower than finger, meaning typing ihello<esc> is FAR faster than <pedal down>hello<let go>, not to mention that pedal travel is way longer.
