You could go the DIY route and build yourself a foot switch. Sustain pedals are momentary switches. You push down and the pedal engages; you release and the pedal turns back off. Effects boxes use a different type of switch, in which you push it once and it engages and stays engaged until you push it again. It should be quite possible to buy a cheap Boss pedal at your local pawn show, strip out the electronics, removing knobs, etc., and just use the foot switch in place of the traditional sustain pedal. Heck, if you've been playing guitar for a while, you might still have one of those pedals you bought when you first started playing that you've stopped using. You might as well use it to experiment with.
I haven't done this, so I'm not making any warrantees that it will work. But at least theoretically it should. The bad news is that you won't be able to use a normal sustain pedal at the same time, without swapping cords. And the sounds won't sustain any more than they sustain while holding the sustain pedal down. (Piano will still decay, etc.) Also, it won't automatically stop and restart when changing chords. But when you're changing chords you're standing in front of the keyboard, right?