Combine the sugar, chopped chiles and 1/2 cup water in a small saucepan. Bring to a boil, reduce the heat to medium and cook, stirring occasionally, until the sugar is dissolved. Transfer to a small bowl and refrigerate until cold, at least 1 hour.
Combine the rum, mango nectar and yuzu juice in a pitcher. Strain the chile syrup into the pitcher; stir until incorporated. Divide among ice-filled glasses. Add a few drops of grenadine to each drink and top with a splash of seltzer. Garnish with a halved chile.