Heat a grill to high (450° to 550°). In a medium saucepan over medium heat, cook rhubarb, ginger, garlic, hoisin, sugar, and 1/4 cup water. When rhubarb starts to break apart, about 3 minutes, whisk in soy sauce and transfer half the glaze to a small bowl.
Thread shrimp onto 4 metal skewers. Brush both sides of shrimp with glaze from pan. Grill shrimp, turning and basting generously as you go, until they're opaque and grill marks appear, about 8 minutes total.
Transfer to a serving plate and brush with glaze from bowl. Sprinkle with onion. Serve with remaining glaze from bowl on the side.