In a pitcher, combine the lime zest, lime juice, Cointreau, tequila and sugar if you are using it. Cover and refrigerate until cold, at least 1 hour).
Just before serving, strain the mixture to remove the zest, and pour enough salt into a saucer to cover the bottom. Rub a lime half over the rim of each champagne glass and upend into the salt to crust it lightly. Pour about 3 ounces of the tequila mixture into each glass, fill the rest of the way with Champagne or sparkling wine and hand to one of your lucky guests.