Preheat oven to 350°F. Place potatoes in a small bowl, and drizzle with oil. Sprinkle with 1 teaspoon of the salt; toss to coat. Spread potatoes in a single layer on a baking sheet, and bake until tender when pierced, about 40 minutes. Remove from oven, and cool 15 minutes.

Cut each potato in half crosswise. Carefully scoop out potato pulp into a medium bowl, leaving shells intact. Place shells, cut side up, on baking sheet, and bake in preheated oven until dry, about 10 more minutes. Cool completely, about 30 minutes.

Stir together potato pulp, sour cream, capers, parsley, dill, mustard, zest, and remaining 1/2 teaspoon salt. Spoon mixture generously into each potato shell.