Measuring heart rate and hacking an exercise machine to extend the grip area
I'm not 100% sure on this but I think the answer is yes.

The sensor would work by measuring the tiny difference in voltage between the left hand and the right hand.

You could prove that retrofitting a larger piece of metal to the poles would work by simply wrapping the poles with aluminium foil. If it still works and successfully measures your heart rate from you touching the foil which in turn touches the existing metal pads, then I think you can safely say that the retrofit would work.