The pineapple-cucumber base can be made up to 6 hours ahead. Add seltzer just before serving.
Muddle mini cucumbers, mint leaves, and 1 cup pineapple juice. Stir in 1 more cup pineapple juice; refrigerate until cold. Pour halfway up ice-filled glasses; top with seltzer. Garnish with cucumber ribbons.
To spike it: Add 1 ounce pisco per serving, before the seltzer.