Combine the water and sugar in a small pot. Bring to a simmer over medium heat, then stir in the rosemary sprigs. Remove from heat, cover and let the syrup steep for 15 minutes. Discard the sprigs, and transfer the syrup to a small jar. The syrup will last for about a month in the fridge.
Fill up to eight glasses just less than half full with champagne. Add two teaspoons of rosemary syrup and two teaspoons of balsamic to each glass. Top with about 1/4 cup of grapefruit juice and garnish with a small sprig of rosemary. (Tip: If you'd like it sweeter, add more syrup or balsamic. If it's too sweet, add more grapefruit juice.)
For a mock mimosa, start with three teaspoons of both syrup and balsamic, and then add your grapefruit juice. Add more syrup/balsamic if you'd like it sweeter!