Set the rack in the middle of the oven and preheat to 450°F Lightly oil one or two roasting pans large enough to hold the eggplant halves in one layer. Add 1/8 inch of water.
Leaving the stem intact, halve the eggplants lengthwise. Use the tip of a paring knife to score the flesh side deeply in a crisscross pattern. Brush the cut side with oil, season with salt and pepper, and set flesh side down in the roasting pans. Brush the skins with oil.
Roast until the eggplants are tender and easily pierced with the tip of a paring kife, 18 to 25 minutes. (Check after 10 minutes and add more water, if needed.) When the eggplants are cool enough to handle, use a paring or grapefruit knife to create a 1/2-inch "wall" all around, and then scoop out the flesh. Coarsely chop the flesh (including seeds) and set aside. Lightly season the eggplant shells with salt and pepper, and reserve them in a warm place.
To prepare the filling, first toast the mustard seeds: Heat 1 tablespoon of oil in a large, heavy saucepan over high heat. Stir in the mustard seeds, cover the pot, and leave the heat turned to high. Listen carefully: as soon as you hear the mustard seeds begin to pop against the lid, turn off the heat (remove to a cool burner if using an electric stove), and wait for the popping to subside. Most of the seeds should now be gray.
Stir the onions into the mustard seeds and cook over medium-high heat, stirring frequently, until the onions start to brown, 4 to 5 minutes. Add a bit more oil during this time if the onions are sticking. Stir in the curry powder and cook for 10 seconds. Then add the tomatoes, chickpeas, coconut, reserved eggplant flesh, salt, and cayenne to taste. Cover and simmer over medium heat, stirring occasionally, until the chickpeas develop a curried flavor, about 15 minutes. During this time, add more curry and salt, if needed, and stir in a few tablespoons of water if the mixture becomes dry.
Mound the filling into the eggplants shells. (If you have a little extra filling, enjoy a snack.) Garnish with a sprinkling of cilantro, if you wish, and serve immediately.