When I was first learning emacs and trying to figure out a sane way to type double bucky[1][2] commands, I happened to read about a project like this for using pedals as modifier keys.

However, instead of using the obvious hardware approach for the time - probably using a couple momentary switch pedals and reading them from the parallel port - I had a much better^Wmore fun idea: use a MIDI organ pedal board[3]. I already had a MIDI interface (Midiman MM-401), and I had already dreamed of getting such pedals for regular keyboard use. Having 1-2 octaves available was plenty of room for all kinds of extra macros and/or hotkeys.

I was half way through writing a simple daemon that would listen for MIDI and send the appropriate synthetic X11 KeyPress/KeyRelease events when I finally looked into where I could buy a pedal board, and realized they were $600+ (in mid-90s dollars). ~sigh~

[1] e.g. "C-M-SPC" for mark-sexp

[2] http://catb.org/jargon/html/D/double-bucky.html

[3] anything similar to this: http://www.nordkeyboards.com/products/nord-pedal-keys-27