Well, it sounds as though you may have greater technical knowledge of this than I do, but it just seems to me that, assuming a standard ballot layout, such as in columns, all that would be required would be to calibrate the machine so that touching a spot would, for example, register far enough to one side to cast a vote for the Democrat. Beyond that, I don’t know if such calibration could be page-specific, working only on those where candidates were displayed (as opposed to ballot propositions and the like).
Your idea of “calibration” is close to what I was saying but there is no adjustment calibration; it all involves the code that registers the touch point and to which “bucket” it applies the vote. That part of the process is all hidden to the voter so if the code has been written to apply every 10th republican vote to a democrat then the voter will never know.