To a certain extent, you have that in existence somewhat today, because merchants can offer a discount for cash. That ultimately is really the biggest break on excessive interchange rates, I think. If all of a sudden MasterCard or Visa tomorrow wanted to charge 10% interchange, I think you'd quickly see merchants offering significant discounts for cash payments.
The model you're describing is surcharging. That could also work as the flip side of a discount for cash. Merchants can compete on whether or not to absorb the cost of interchange, as they absorb the cost for shipping or other features they offer their customers.
But I think you're on the right path when you say let the market be transparent and let it compete.