Step 1Combine the sugar, water, white wine and lemongrass in a saucepan. Bring to a boil. Lower the heat and gently simmer, uncovered, for 20 minutes. Turn off the heat and let the mixture steep for at least 1 hour. Strain and set aside.
Step 2In a blender, combine the watermelon, peach, cantaloupe and half of the blueberries. Puree until smooth. Strain the mixture through a chinois or fine sieve (if you have a juicer, you can use it instead of a blender and skip this step because a juicer will strain the fruit.) Add the orange juice and some of the lemongrass simple syrup to taste, about 1 to 2 tablespoons. Chill the mixture.
Step 3Just before serving, cut the remaining blueberries in half and place in a small bowl. Add the mint, ground cayenne pepper, a pinch of salt and 1/4 teaspoon of Cointreau. Stir gently and divide among 8 demitasse cups.
Step 4To the watermelon mixture, stir in 1 1/4 teaspoons Cointreau, the Champagne and club soda. Divide the mixture among the cups, pouring over the blueberries. Serve immediately.