Preheat the oven to 375 degree F. Use a long, heavy knife or kitchen shears to halve Cornish hens lengthwise, cutting through the breast bone, just off-center, and through the center of the backbone. (Or ask the butcher to cut hens in halves.) Twist wing tips under back. Rub the olive oil over the surface and in the cavity of each hen half, then sprinkle generously with salt and pepper. Place hen halves, cut side down, in a 15x10x1-inch baking pan. Roast, uncovered, for 40 minutes.
Meanwhile, for sauce, in a medium saucepan bring the rhubarb, barbecue sauce, and water to boiling over medium-high heat. Reduce heat to medium-low and cook, covered, for 20 minutes or until the rhubarb loses its shape. Remove from the heat and coarsely mash the rhubarb in the pan. Remove and reserve 1 cup of the sauce for the orzo mixture.
Brush the surface of the game hens with some of the remaining sauce. Roast for 15 to 20 minutes more, brushing every 5 minutes, or until hens are glazed, tender, no longer pink, and an instant-read thermometer inserted in thigh muscle registers 180 degree F.
Meanwhile, cook orzo according to package directions; drain. Transfer orzo to a large bowl. Stir in the 1 cup reserved sauce, shredded carrots, and green onions. Season to taste with salt and pepper. Serve orzo mixture mounded on a large platter and topped with game hen halves. Pass the remaining sauce. Makes 6 servings.