In a large bowl, combine the rice and enough water to cover by 2-inches. Soak for at least 6 hours or up to 24 hours. Drain.
Place a bamboo steamer inside a wok or large pot. Line the steamer with parchment paper. Add enough water to the wok or pot to come up just below the bottom steamer rack, 1 to 1 1/2 inches. Bring the water to a boil. Add the rice to the bottom steamer rack, pour 1 cup of boiling water over the rice, reduce the heat to medium-low, cover, and steam until rice is tender, 30 to 40 minutes. Remove from the heat and transfer the rice to a bowl. (At this point the coconut sauce should be made and ready to pour over the steamed rice.)
While the rice is steaming, prepare the mango syrup and the coconut sauce. In a small saucepan, combine the diced mango, lemon juice, water, and 1/2 cup of the sugar. Bring the mixture to a boil. Reduce to a simmer and cook for 5 minutes or until the fruit is very tender. Pour the mixture into a food processor and puree until smooth. Remove and chill the mango mixture for 30 minutes.
To make the coconut sauce, combine the coconut milk, remaining 1/2 cup sugar, and salt in a medium saucepan over medium-high heat. Bring the mixture to a boil, and cook, stirring, until the sugar dissolves, about 1 minute. Pour over the steamed rice, mix well, cover and set aside until most of the liquid is absorbed, 30 to 45 minutes.
To serve, spoon 1/2 cup of the rice pudding into each of 6 shallow bowls. Arrange strawberry slices on top of the pudding and drizzle the mango syrup over the top of the pudding. Garnish each with mint sprigs and serve at room temperature. (This dessert is best before it has been refrigerated.)
Notes*Thai black rice can be found in most Asian supermarkets. If unavailable, glutinous white rice can be substituted, but won't have the nutty flavor of the black rice. One cup of dried black rice yields 2 cups of cooked rice.
Cook's Note: When using a bamboo steamer and a wok for steaming, we place marbles (or ceramic pie weights) in the bottom of the wok with the water. When the marbles begin to rattle, add more water to the bottom of the pan. This helps to keep the wok or pan from scorching the bamboo steamer.