The Third Hand, by Tone in Progress is a rocker foot pedal attached to a extension tube which connects directly, physically, to a knob on another effect pedal. So any control can be tweaked with the pedal. It's a tiny, delightful bit of mechanical genius for £95. I don't know if you can get longer cables, which could snake up to a modular synth like some kind of steam-powered high-tech pedal steel. I discovered the 3rd Hand on the site of Jam Guitars, a fantastically boutique guitar shop in my home town of Bristol (yes, England).
UPDATE: Looks like it was inspired by the Electro Harmonix Hot Foot, which does pretty much the same thing.