Measure reserved liquid from tomatoes and chicken together; pour into a bowl. Add enough water so the liquid equals 3 cups; pour into rice cooker along with chicken broth, tomatoes, rice, lentils, spinach, mushrooms, basil, onion flakes, soy sauce, garlic powder, salt, and pepper.
Cook in rice cooker until the "done" signal chimes and the rice is completely tender, 60 to 90 minutes.
