In a pot, combine all the tamarind, palm sugar, lemongrass and 2 1/2 cups water and bring to a boil. Mash the mixture with a potato masher. Strain the stock through a fine-mesh strainer lined with cheesecloth while pressing with the back of a wooden spoon. (This can be done 4 to 5 days in advance.) Keep refrigerated until needed.
Purge the clams: In a bowl, combine the clams, cornmeal, and 4 cups of cold water. Place in the refrigerator for about 30 to 40 minutes, to clean out any sand. Remove the clams from the water and discard the water.
Pass the tomatoes through a food mill to remove the seeds (they can be bitter), so that only the juice remains. Set aside.
Bring a pot of water to a boil and cook the cellophane noodles for 4 to 6 minutes, or until done. Strain and put 1/2 cup each of the noodles in 4 wide soup bowls.
In a wok fitted with a steamer rack (or circular cake rack), bring the tamarind stock, ginger juice, and tomato juice to a boil. Place the clams on the rack, cover, and cook until they open and release their juice into the broth. Divide the clams among the soup bowls, leaving the broth in the wok.
Stir the sweet soy sauce, minced chile, and basil into the broth and simmer for 1 minute. Remove from the heat and stir in the fish sauce. Ladle the broth over the clams and garnish with scallions and lemon zest. Serve immediately.