Add the grapefruit juice, maraschino cherry juice, pineapple chunks and li hing powder, if using, to a blender. Blend on high until smooth.
Divide the juice among 8 to 10 large glasses and add ice. Divide the seltzer among the glasses. Float with 2 ounces of dark rum if desired. Top with a wedge of pineapple and a maraschino cherry if using.