Sprinkle gelatin over water in a small microwave-safe bowl; let stand 1 minute. Microwave at high 15 seconds, stirring until gelatin dissolves; cool slightly.
Beat egg whites at high speed of a mixer until soft peaks form.
Combine sugar and lemon juice in a saucepan; cook over medium heat until sugar dissolves. Increase heat to medium-high, and cook, without stirring, until candy thermometer registers 24
Pour hot sugar syrup in a thin stream over egg whites, beating at medium speed, then at high speed until stiff peaks form (about 5 minutes). Beat in gelatin mixture, grated lemon rind, and lemon extract. Fold in whipped topping, yogurt, and salt. Spoon 3/4 cup lemon mixture into each of 8 (6-ounce) custard cups or ramekins. Cover and freeze 4 hours or up to 24 hours.
