This electronic foot switch could be used if you only have +9 to 15V in the effect.
The switch is built round three Field Effect Transistors, which I think don't change 
the sound too much. The IC is a dual JK flipflop in CMOS technology (4027). The FET:s 
can be BF 245A,B,C or BF 256A,B,C.
                                                    
If you want a LED indicator to show when the effect is in or bypassed you can add a 
470 kohm resistor to the inverted output of 4027. Let it go to for example a BC 548 B 
transistor's base. The emitter would go to ground and the collector to a LED and a 
2.2 kohm (or related value) resistor in series to +V.