I think the leopard is a separate dominant (or codominant though not sure if leopard x leopard has been done yet) mutation that is just coincidentally tied in with the pied mutation due to the mutation's origins. Greg Graziani had leopards, leopard pastels, leopard spiders (all het or possible het pied) and a couple leopard pieds, leopard pastel pieds, and leopard spider pieds last year. I think he'd probably be one of the better people to ask about the mutation.