Preheat oven to 375 degrees F (190 degrees C). Line the bottom section of a broiler pan with aluminum foil, top with the broiler rack, and spray rack with cooking spray.
Place a water chestnut slice atop each pineapple cube; wrap each with 1 bacon slice, securing with a toothpick. Arrange wrapped pineapple on the prepared broiler rack.
Bake in the preheated oven for 7 minutes; turn and continue baking until bacon is almost crisp, about 8 more minutes. Brush rumaki with sesame-ginger dressing and continue baking until bacon is crisp, about 5 more minutes. Garnish rumaki with green onion.