Bring 8 cups water, salt, and 3 tablespoons sugar to a boil in a 3- to 4-quart heavy saucepan, then add tapioca in a stream, stirring constantly. Reduce heat and briskly simmer, uncovered, stirring occasionally, until tapioca is almost completely translucent, 35 to 50 minutes, depending on size of tapioca.
While tapioca is cooking, simmer 1 1/4 cups rhubarb, 1/3 cup sugar, pepper, a pinch of salt, and remaining 1/3 cup water in a 2-quart heavy saucepan, uncovered, stirring occasionally, until rhubarb is just tender, about 8 minutes. Purée in a food processor until smooth (use caution when blending hot liquids), then reserve 1/4 cup (to add to tapioca) and return remainder to saucepan. Stir in raw rhubarb and simmer, stirring occasionally, until tender, about 5 minutes. Stir in lemon juice, then transfer to a shallow bowl.
Drain tapioca in a sieve and transfer to cleaned 2-quart heavy saucepan. Stir in half-and-half, remaining 2 tablespoons sugar, vanilla, and reserved rhubarb purée and bring to a simmer, stirring occasionally until sugar is dissolved. Cool slightly, then divide among 4 dessert bowls and top with rhubarb.
Each serving contains about 248 calories and 2 grams fat.
