well, like you said, part of the problem is that most foot pedals are toggle switches, you either toggle them on or off, and they stay that way, they usually aren't like pushbuttons, however some are.
If you can find one that acts like a push button, it'd be easy,
Open up a keyboard, take out the little flap with the chip on (it'd be better if you could find an old keyboard that doesn't have it's pcb printed on floppy plastic) with the keyboard wire attached to it,
follow the lines on the pcb to the spacebar to see how they are hooked up to the chip, reproduce it with some wires and hook it up to the switch, then if you have a hard pcb, saw off the rest of the keyboard pcb, or if you have a floppy pdb you could maybe get away with just hooking up the wires to the switch straight up to the space bar contacts, and rolling up the flap and sticking it in the pedal. However you might have some trouble soldering/connecting wires to a floppy pcb flap like that, which is why i'd just hook em straight up to the chip and examine the pcb to see if it needs adittional connections.
If the switch is the stay on-stay off type, you can make a logical circuit that monitors a change in the state, and outputs a pulse like a normal pushbutton everytime the state changes. I'd have to figure that out in circuitmaker
first or something, then when i have my schematic get the needed IC's from radioshack, and build it on a little breadboard
, test it, if it works, cool, then solder it to one of those grid pcb things
that they have, which you can cut to size. If you're doing that you might as well put the keyboard chip on the little breadboard pcb too.
I'm not sure if I would mess with the poor marshall and hook it up to a transistor box like a PC
The nice sound from the marshall comes from it's tubes, keep it far far away from transistors