It seems like it should be pretty easy to do, right? You shouldn't need any electronics: just a momentary switch, some wire, and a 1/4" jack and instrument cable, or just wire the cable directly to the switch, and an enclosure to put it all in, if I'm not mistaken. Don't even really need to do any soldering.
I've made a two-button foot-switch for my guitar amp using this method, and it's really simple and cheap.
Anybody here ever done such a thing?