Preheat charcoal grill. Skewer 2 cod fillets on two metal skewers, about an inch apart, to hold fish pieces securely. Repeat with remaining cod. Cook fish skin side down over hot grill for 3 to 4 minutes until skin is charred and crispy. Then turn over and cook another minute or two, until just cooked through.
Heat saute pan over high heat and add oil to coat bottom. Add ginger, garlic, and maitake mushrooms. Then add the sausage and cook, breaking up the meat with a metal spoon, a minute or two. Add the sake and cook a minute then add clams. Cook just until clams open up, about 6 to 7 minutes. Add soy sauce. Taste for seasoning.
To plate, spoon clams with sake broth into serving bowls. Place cod on top and garnish with kumquat slices and scallions.