I've tried this for spacebar, but the experiment didn't last long.  Was probably worried that it would effect my posture.  In that it would be better for me to have both feet on the floor.

Though I once could write with my feet, just about, but have no proof - just my word!

Left foot can press space, right foot can press backspace. Pretty nice idea.

@pinkyache
That spacebar is probably the best BUTTON for this task. It is BIG. Personally I don't see much point in a spacebar footswitch though, as my right thumb has nothing better to do anyways. Also feet are slower than fingers. And there is a posture issue that I'll cover later, so hold that thought.

@DreymaR
Ok, I'll admit I didn't actually try this out until that pic request came in. I was actually trying to find a way to re-purpose an extra mouse into left thumb button. That was awkward though, so I tried to find a way to have a trackpad touch register as a keystroke instead of a mouse click. Like the Maltron E, for example. Problem is that they are both bound as pointing devices, and there is no easy fix to distinguish inputs from two similar devices. Like keyboards. 2+3=1.6 and there you have it. Footswitch. (I forgot to mention that I had briefly considered racing pedals)

Bigger keys would be nice... BUT (there's always a but) there are concerns. Dome keyboards don't have the most stable key action to begin with. Increasing the surface area of the key increases the likelihood that you would press it off center and the key would bind. While you could force through a bind with your foot easily, there is no need to actively try to break the junk board. Bigger keys also decreases distance and distinction between keys and increase the chances of hitting the wrong one. Out of curiosity I did try swapping a shift key around to a letter position, but the adjacent columns prevent the key from fitting without removing the sides of the key, and even then the large key stabilizer clip has nothing to clip into. So, in general, it is not worth the trouble, my feet are big enough to hit whatever keys are under them.

You could try making them higher or increasing grip by gluing some rubber pads or washers to the top, but if you are pivoting your foot from a rest position down the center or off the side of the board the extra height will cause more work. This would still be a good option for scissor switch keys though. There the low profile works against them, and the added height gives them a bit of contrast. Scissor switch domes are not as well protected however, so you would need to cover the stripped part of the board with something like cardstock and packing tape.

As for grip, junk keyboards are light, increased traction on the keys will probably cause more problems than it will solve.

Tried somethings out, so I'll post some tips further in.

@Tony_VN
I'm not too keen on having delete or backspace on the floor. You know, where a dog/cat/child might sit on it. Or one of my two left feet might hit it. Backspace also tends to require several quick successive strokes, which feet are poorly suited for. And as for the spacebar, again, my right thumb has nothing better to do than hit the spacebar. Unless my hand is on the mouse, at which point if for some odd reason I need simultaneous mousing and spacing action, I always have my LEFT thumb.

Considerations

With the recommended feet flat on the floor, working the footswitch with the ball of the foot can be unnatural. The keys are not meant to hold the weight of a resting foot, and most of the ways around that (pivoting/hovering) use the wrong muscles. I don't think any of us have the desire to become the first recorded case of ankle RSI. Playing around with the, heh, prototype, I have had good results with placing the board so that you can strike the keys with the heel and pivot to and from a resting position or even hover on the ball. This should give greater leverage, cause less strain and, if hovering is required, be less tiring. (I did this with spacebar side of the board closest to the monitor, balls of my feet on the floor, heels resting over/on the space and numberpad)

If you use the heel method on a wheely chair, you might have to give up all but the L.Shift or L.Ctrl and place the board lengthwise under your chair. Or maybe backspace and R.Shift or R.Ctrl.

Feet are slower than fingers. No joke.

You will not be able to take your footswitch everywhere you go. Do not develop habits that will cripple you without it. For instance, don't use it for the letter E.

Practical footwear. Boots are probably a bad idea. Loose fitting loafers might also cause problems.

Mind the gap. While two keys close together might seem ideal for one foot, this doesn't leave much room for a resting position. If you rest to the side then you need to successfully clear the first key before stomping on the second. A dicey maneuver considering most people don't have the best foot to keyboard coordination. With only some minor testing to go on, it seems wiser to me to have buttons pressed by the same foot to the sides. For instance (UPDATE), L.Ctrl AltGr on L.Foot, R.Shift and N.Enter on R.Foot. Again, using the heel seems to give wider angle, with less strain.

With programs like KeyTweak you CAN remap a key (usually at the registry level) such as the Numberpad Enter into something more useful, but be warned: the changes will effect BOTH the footswitch AND the keyboard. There is no way I know of to make your computer differentiate between keystrokes on different keyboards. So don't get rid of a key you regularly use. Also, much like a keyboard layout the changes are done on the computer, not the board. If you move your footswitch to a different computer you will need to make the change again.

Update: Further fiddling finds footswitch farther forward fine, fixes flat-footed's foul front flexing. In other words, if you want to press the keys with the ball of your foot, put the footswitch farther away. This naturally causes the front of the foot to pull away from the floor and... yeah, this is pretty self-explanatory. Just play around with it yourselves. If you find something interesting, post it. For instance, I recently discovered that using the footswitch for crouching or jump in a FPS is oddly immersive.

Also, for what it is worth the $15 estimate was used because it rhymed. You can probably do it for less. On the flip side proper footswitches, based on amazon prices, run from around 13-90 for single key, to 50-300+ for triples. Granted they all have better programmability.

Note: Sorry about frequent updating and editing. Check time below to see if you've read the latest version.