Just an idea - how about modifying a front derailer by removing (or breaking) the tension spring and tightening up the pivot points somehow so that it does not flop around and stays put in one position. This way you could manually move the derailer inwards or outwards before starting the ride to switch the front gears. The FD is light enough by itself and your hands will stay clean because the derailer will do pushing the chain up or down the chain rings.