Heat the oven to 350° and spray an 8-inch round or fluted cake pan with nonstick baking spray. In the bowl of a stand mixer fitted with a whisk, beat the egg whites and cream of tartar on medium-high speed until soft peaks form. While whisking, slowly pour in 3 tablespoons granulated sugar and continue beating until stiff peaks form. Using a rubber spatula, scrape the beaten whites into a large bowl.
Add the egg yolks, the remaining 4 tablespoons granulated sugar, and the salt to the bowl of the stand mixer and whisk on medium-high speed until it triples in volume, turns pale yellow, and falls in a thick ribbon when lifted from the whisk attachment. While the yolks beat, remove the zest of the pomelo with a Mircoplane zester. Add the pomelo zest to the beaten egg yolks along with the mescal and oil and whisk until smooth. Pour over the beaten egg whites, sprinkle in the flour, and fold until evenly combined.
Scrape the cake batter into the prepared pan and smooth the top. Bake until golden brown and a toothpick inserted in the middle of the cake comes out clean, 25 to 30 minutes. Transfer the cake to a rack and let cool completely in the pan. Unmold the cake onto a cake stand and dust with confectioners' sugar.
While the cake bakes and cools, use a paring knife to cut away the white pith of the pomelo, then cut between the membranes to release its segments into a bowl. When ready to serve the cake, cut it into wedges and serve with a dollop of whipped cream and pomelo segments.