Look up Whitebone taxidermy on YouTube. He has done some pheasant, etc. Best way to do it would probably be to skin the head before boiling and maybe even cut the entire bill off and epoxy that to keep it preserved (or soak it in a solution of alcohol and glycerin). One the rest of the skull was bleached you could reattach the bill.
Otherwise, boil it all and then paint the bill to match what the duck used to be.