In a small saucepan, combine 1 cup sugar, 1 cup water, and lemon zest. Bring to a simmer over medium heat, stirring until sugar is dissolved. Remove from heat; let cool.
Place four 3-by-3-inch ring molds on a parchment-lined baking sheet. Using a pastry brush, lightly coat ladyfingers on both sides with cooled lemon syrup. Trim 1 1/2 inches from the end of each ladyfinger. Using short pieces, flat sides down, form a base, trimming if necessary to fill completely. Place long pieces, flat sides in and cut sides down, around the perimeter of each mold.
Fill each mold with 1/2 cup sorbet, pressing with a small rubber spatula to fill any gaps. Wrap molds tightly with plastic wrap, and freeze overnight.
Just before serving, with an electric mixer, beat egg whites and salt on medium-high until soft peaks form. Gradually add remaining 2/3 cup sugar, one tablespoon at a time; continue beating until stiff and glossy.
Remove charlottes from freezer; let stand for 5 minutes. Discard parchment paper. Carefully press up with your fingers from bottom of each charlotte to release. Dollop each charlotte with meringue. Using a small blowtorch, brown meringue; alternately, place under broiler and heat until golden. Transfer to serving plates; serve immediately.