Combine 1/2 cup each granulated sugar and water in a medium saucepan and cook over medium heat until the sugar dissolves, about 4 minutes. Remove from the heat. Add the jalapeno and set aside 15 minutes to infuse the syrup. Remove the jalapeno and discard; set the syrup aside to cool completely.
Pour the syrup into a blender and add the pineapple and tequila. Blend until very smooth, about 2 minutes. Transfer to a pitcher; refrigerate until ready to serve, at least 1 hour. Moisten the rims of 4 to 6 glasses with lime wedges, then dip in demerara sugar; fill with ice and the cocktail.