Here's the mouse essentially de-moused. I desoldered the buttons, the rotary sensor for the wheel, removed the wheel mount, and even the LED for the optical sensor (I left the power LED though.) I stuck a piece of black tape over that optical sensor just to prevent it from getting confused. A ball mouse might have made things simpler, but this was what I had that met all my requirements.

The mouse buttons were these little push switches. There's no polarity or circuitry in these, they just short when closed. Perfect. (These went in the parts bin for re-use, naturally.)

This will be our project box for this, er, project. It turned out to be just about perfectly sized. No idea where I got it.

Threaded my own cable into the foot switch and connected it up. This was actaully a motherboard USB cable, but I'd long since lopped off the USB port and used it in something else. Lucky I never throw stuff like this away. Importantly, it had enough conductors, was shielded (which probably doesn't matter in this application), and the other end was broken off into pushpins.

Some fan headers harvested off a dead AM2 board turned out to have the perfect spacing (with the middle pin removed) to neatly replace the mouse button switches.

Those former fan headers let me re-use the quick connectors on the ex-USB cable:

Here's the "final" assembly before I closed everything up. I ended up hot gluing the board down to a suitably-cut piece of stiff card stock to keep it in place, using some motherboard standoffs through the existing screw holes as feet. This isn't the prettiest thing in the world, but it's a DIY build and will do the job, so who cares. I used a bunch more hot glue on the cables for strain relief since I didn't have proper grommets to go through the sides of the box.

Gave it a quick test before I closed everything up & called it a night - worked perfect!

Because my foot pedal is now "a mouse", it works on essentially any OS out there - Windows, DOS, Linux, OS/X, BSD, Haiku, whatever. Who needs custom drivers? In fact, because the mouse I chose was PS/2 compatible using an adapter, I can even plug it into non-USB systems and use it for DOS gaming (with the caveat that you can't have two PS/2 mice simultaneously, so you only get the buttons.)

Here's the final assembly:

(And yes, I know they're foot pedals, and they're supposed to go on the floor, rest assured that's where they went after I took the picture. You guys don't need to see my carpet.)

Note that it's plugged into the PS/2 port & is running on pure DOS here via CuteMouse:

You can use this for any DOS game or program that supports mouse buttons for input! I was hoping Descent would support using the mouse & joystick simultaneously, but it makes you pick one or the other. However, ROTT DOES let you do this, so I played through a few maps this way, which worked surprisingly well. I'm sure you can do this in other stuff too. Most Windows games will happily let you use a mouse alongside other controllers, and Windows/Linux/etc. support as many mice as you have USB ports.

I mapped the left mouse button to the right foot switch, RMB to the left foot switch, and middle-to-middle, which made sense in my head. I can easily switch it around if I decide to. The one I use on my Win7 machine actually has LMB, X and C mapped to it in software, but just getting the three mouse buttons with no extra drivers needed is just as good in my book. If you wanted to modify a mechanical keyboard, you could make the foot switches work as keys too, but I didn't feel the need to here.

Also note: if you find a USB one that you can't get drivers for, nothing's stopping you from ripping out its USB control board and wiring the switches themselves up however you like. Those might even have enough room to mount an ex-mouse PCB internally (this one didn't.)

Overall, SUPER happy with this little day project turned out. I hope this post inspires some of you & saves some formerly-expensive foot pedal sets from the E-waste pile.