Heat oven to 325° F. Grease and flour a 2-quart ovenproof metal mixing bowl. In another medium bowl, combine the cake mix, eggs, oil, and lemon zest, along with the amount of water called for by the package directions. Beat until well combined. Pour the batter into the prepared bowl. Place in oven and bake until a knife inserted into the cake's center comes out clean, about 50 minutes. Let cool 20 minutes before unmolding onto a rack.
Wash and dry the bowl you baked the cake in. Line it with plastic wrap and place the cake back in the bowl (support the bottom so the dome doesn't break). Using a skewer, liberally pierce the cake, poking through to the bowl. Pour the lemonade concentrate over the top. Let the liquid soak in for 15 minutes before unmolding the cake upside-down onto a platter. Just before serving, dust lightly with the confectioners' sugar.