In a small saucepan over medium heat, stir together the sugar with 1/2 cup water and the ginger slices. Bring to a simmer to dissolve the sugar, then remove from the heat and allow to cool to room temperature.
In a large pitcher, stir together the ginger simple syrup, pineapple juice and seltzer. Pour into ice-filled glasses and add 1 1/2 ounces of rum to each glass, if using. Garnish each with a pineapple chunk.