Put the scallops and mushrooms in 2 small separate bowls and cover with water. Let stand at room temperature overnight.
In a small saucepan of boiling salted water, cook the corn kernels until just tender, about 2 minutes. Drain.
In a large saucepan, combine the chicken broth, corn cobs, chicken thigh, ginger and sliced celery and bring to a simmer. Cover partially and simmer over low heat until the chicken is cooked through, 30 minutes. Strain the broth and reserve the chicken thigh. You should have about 5 1/2 cups of broth.
Preheat the oven to 325°. Measure out 1 cup of the broth and reserve. In a 3- to 4-quart casserole or Dutch oven, combine the remaining 4 1/2 cups of broth with the rice and bring to a simmer over moderate heat. Cover and bake for about 20 minutes, stirring occasionally, until the rice is tender and a thick porridge has formed.
Drain the scallops and transfer to a small saucepan. Add 1/2 cup of the reserved broth and bring to a simmer. Cover and simmer over low heat for 10 minutes. Using a slotted spoon, transfer the scallops to a plate. Remove any membranes, then thickly shred the scallops; cover. Add the tamari to the cooking liquid in the pan and cover.
Drain the mushrooms and cut off the knob ends. Cut the mushrooms into 1/2-inch pieces. Remove the meat from the chicken thigh and cut into 1/2-inch pieces. Add the chicken, mushrooms, corn, the remaining 1/2 cup of reserved broth, half of the celery leaves and the cheese to the porridge. Season with salt.
Spoon the porridge into bowls. Top with a little of the scallop-tamari broth. Scatter the shredded scallop and remaining celery leaves over the porridge and serve.