Position a rack in the center of the oven and heat the oven to 350°F.
Spread the coconut in a thin layer on a large rimmed baking sheet and toast in the oven, stirring twice, until dark brown, 6 to 8 minutes. Transfer to a plate to cool. Put the bread cubes on the same baking sheet, spread in a single layer, and bake, stirring halfway through, until a pale golden brown, 15 to 20 minutes total. Let the bread cool on the baking sheet. (Turn the oven off.)
Meanwhile, in a 2- or 3-quart saucepan over medium-high heat, bring the half-and-half, coconut milk, 2/3 cup of the toasted coconut, the lemongrass, and a pinch of salt to a simmer. Remove from the heat, cover, and let stand for 45 minutes. Pour through a coarse strainer set over a medium glass measuring cup or bowl, pressing hard on the solids to extract as much of the liquid as possible.
In a large bowl, whisk together the eggs and sugar. Slowly pour in the warm half-and-half mixture, whisking constantly. Toss in the bread and push it down with a spoon to submerge it. Let stand for 30 minutes, or until the bread is softened. Butter eight 8-fl.-oz. ramekins. Divide the bread mixture evenly among the buttered ramekins and pour any remaining custard over the top of each.
Heat the oven to 325°F. Have ready a roasting pan or baking dish large enough to accommodate the ramekins. Put on a kettle of water to boil for the water bath.
Set the ramekins in the roasting pan and put it in the oven. Carefully pour enough boiling water into the pan to come halfway up the sides of the ramekins. Bake until the custard is set, 30 to 35 minutes. (To check, cut into a pudding with a paring knife and peek into the center—the custard shouldn’t be runny.) Carefully remove the ramekins from the pan and let them cool slightly on a wire rack.
Sprinkle the puddings with the remaining 1/3 cup toasted coconut and serve.