Update 2018-07-20: my wrists have calmed down some and I'm worried about hurting my knee, so I've switched to mousing lefty but still "clicking" by using keys on the keyboard.

Update 2019-11-07: about a year ago I switched to using my laptop full time, with an external monitor. The trackpad can be used from enough different positions that my wrists have stayed just (barely) this side of ok.

I've been having wrist issues that seem to be especially bad when using the mouse, so I decided to try using the mouse with my toes. There have been various attempts to make special-purpose mice for use with your feet, but I figured a good place to start was with the mouse I already have:

Here's what I needed to do:

1. Make a way to click from the keyboard. I set xbindkeys up so that Print Screen / Scroll Lock / Break are the left / middle / right mouse buttons.

   For press-and-hold, like with click+drag, I couldn't get xbindkeys to properly handle Release, so when I held down a key I got press-release in quick succession. Someone else seems to have had the same problem (stack overflow) and I decided to deal with it by mapping F11 to mousedown and F12 to mouseup, all for the left mouse button which is the only one I need to click and drag with.

   For the scroll wheel I decided Home / End was nice since they're right next to Page Up / Page Down and I don't use them normally. Having keys for small scroll movements is wonderful regardless of the rest of this setup.

   My ~/.xbindkeysrc looks like:

   "xdotool click 1"
     Print
   
   "xdotool click 1"
     Insert
   
   "xdotool mousedown 1"
     F11
   
   "xdotool mouseup 1"
     F12
   
   "xdotool click 2"
     Scroll_Lock
   
   "xdotool click 3"
     Pause
   
   "xdotool click 4"
     Home
   
   "xdotool click 5"
     End
   

2. Make a way not to click from the mouse. As hard as intentionally clicking mouse buttons with your toes is, not accidentally clicking is just as hard. I turned off all the buttons with:

   xinput set-button-map \
     "Logitech USB Optical Mouse" \
     0 0 0 0 0 0 0 0 0 0 0 0
   

Long term this would have me worried about my knees, since I do also have occasional knee issues. I'm trying to deal with this by (a) mostly using my toes and not my whole foot and (b) switching back to hand mousing some once my wrists are feeling better.

(Looking at other ways of using your feet for mousing, like the flip flop mouse, boomer mouse, or this homemade one, none of them seem to take advantage of how agile toes are. They're all whole-foot devices.)