Well, after thinking about it a bit more carefully, it shouldn't interfere with the piezo output, so long as you parallel connect the piezo's +ve wire AFTER the resistor on the switch line wire, not before, so that the output of the piezo doesn't have to go through the resistor to go out of the cable. It then shouldn't diminish the output. And then you won't need a DPDT switch to change modes - just plug it in and start using it.
HOWEVER, connecting the piezos to the super drums wiring in parallel MAY interfere with the pads when used for the original function for super drums keyboards; because here, you are wanting to use the rubber mats as mechanical switches, and you have a piezo element bridging that rubber switch permanently already. Piezos have very high resistances and I should think that they would probably not interefere too much with the keyboard drum signal that is being switched by the rubber pads, but try it out in super drums mode to see. If you get erattic firing of your super drum sounds then you may have to put in the dpdt switch to stop the piezos interfereing with them.
cheers,
Graham