We took a stab at copying the Agra Hard Seltzer from More Brewing in IL.
For fermentation, we used EC1118 champagne yeast and 2 packets of Propper Seltzer yeast nutrient from Omega. This fermented really dry. Dropped it from 1.045 to 0.991 on the gravity.
After fermentation, we added 9lbs of Blackberry puree (Oregon Fruit Purees, (3x) cans) and then .005% of Amoretti Meyer Lemon extract to the keg. We also used half a camden tablet and Potassium Sorbate to try and keep the yeast from fermenting the fruit. We're estimating that FG after fruit addition was 1.002 but never actually checked. For the meyer lemon extract, that addition was based on weight in the keg. We probably could have gone with a bit more of that but we didn't want it to overpower the blackberry.
The whole concept behind the Agra seltzer at More was to be different. it's not your typical seltzer. It was meant to be thick and fruity with a nice dry, champagne finish. While not your typical seltzer, this was fantastic. I'm not sure if we'll go the route of adding the fruit to the keg again, as it was a BIT difficult to pour at times. But definitely recommend.