To make the sauce: Heat oven to 400 degrees. Spread the bones in a saute pan and roast until the bones are brown and the fond at the bottom of the pan is beginning to darken, about one hour.
Remove the pan from the oven and put on a stovetop. Deglaze the pan with about 1/3 of the sake, scraping up the browned bits until the bottom of the pan is clean. Then add in the rest of the sake, mirin, sugar, and soy sauce. Bring to a boil, then simmer until the sauce is thickened, about one hour. Season with black pepper. Leftover sauce will keep in the refrigerator indefinitely if reheated once a week.
To grill: skewer chicken hearts and set over a medium high flame.
When the chicken hearts are almost cooked through, about 2 minutes, baste with the sauce and grill again until sauce is dried, about 20 seconds.
Baste again and grill just until the sauce is starting to dry, about 10 seconds.
Sprinkle with sansho pepper if desired. Serve with wedges of lemon, salt, and mustard on the side.