Mix flour and a pinch of salt together in a medium mixing bowl and set aside. Put egg into another medium bowl and lightly whisk, then add beer, olive oil, and lemon zest and whisk until well combined. Add egg–beer mixture to flour and whisk until batter is smooth. Cover and set aside at room temperature to let rest for 1 hour.
Pour peanut oil into a medium heavy-bottomed pot to a depth of 2" and heat over medium heat until temperature reaches 370° on a candy thermometer. Meanwhile, rinse oysters in cold water, put them into another medium pot, and add enough of the reserved oyster juices that they reach about halfway up oysters, adding a little water if necessary. Bring to a simmer over medium-high heat and cook just until oysters contract and plump up, about 3 minutes. Drain oysters.
Give batter a quick stir. Working in 2 batches, use your fingers to dip one oyster at a time into batter, then carefully drop oyster into the hot oil. Fry oysters, gently stirring and turning them with a slotted spoon occasionally, until batter is crisp and golden, 1 1⁄2–2 minutes. Transfer to paper towels to let drain. Season to taste with salt while still hot.
Serve oysters in clean, dry oyster shells with a squeeze of fresh lemon, if you like. Serve immediately.