Pour pureed pomelo and lemon juice into a pie plate. Stir in lemon zest and cooled syrup until combined. Place the dish, uncovered, in the freezer until mixture is just starting to freeze around the edges but still slushy in the middle, about 45 minutes. Use a fork to scrape the mixture from the outer edges inward. Place back into the freezer, and continue to scrape with a fork about every hour until the granita is light and fluffy.