Remove and discard the lemon peel from the chilled syrup. Combine the syrup and the raspberries and place the mixture in batches in a blender. Process each batch until smooth. Strain mixture through a fine-mesh strainer, pressing to extract all of the juices. Discard seeds and any remaining pulp. Stir Champagne and lemon juice into the raspberry mixture until combined.
Pour the raspberry mixture into an ice cream machine and churn according to the manufacturer's directions. (This sorbet may take a little longer because of the alcohol content.) Remove sorbet from the ice cream maker, place in freezer-safe containers, and freeze until firm, about 8 hours.
For the floats: Place 3 tiny, tablespoon-sized scoops of the Raspberry Sorbet into each Champagne flute. Slowly pour the Champagne over the sorbet. Garnish each glass with fresh raspberries, if desired. Serve immediately.