If you're making these margaritas for a crowd, go ahead and puree a whole honeydew and keep it in a pitcher next to the cocktail shaker so you can continue to make additional margaritas.
In a blender, add the honeydew and blend until smooth. Pour the puree into a large cocktail shaker. Add the lime juice, orange liqueur, tequila and agave syrup.
On a small plate, combine the sugar and salt. Rub the rims of two rocks glasses with a lime wedge and dip the rims into the sugar mixture to adhere. Add a handful of ice cubes to the shaker and shake until very cold (about 15 seconds). Fill the sugar-rimmed glasses with ice, strain the cold margaritas into them and serve.