Reading your followup comments, Chris and Brian (sorry I can't seem to reply directly) - I'm using Pusser's Navy rum, so maybe that's why the tangier storebought juice tastes right to me. Tried once with a Demerara and storebought and it was good, but something was missing. I like the punch of the Navy rum for this one. Thanks for your tips.
The Kentucky Derby is tomorrow, May 6! Can you please bump this to the front page temporarily? It can be side by side with Cinco de Mayo and Coronation Day. :)
Given the limitations of the Soggy Dollar's island location in the 1970s, I am certain this drink was developed with standard bar ingredients of the time, including canned pineapple and orange juice.
Fresh juice is too sweet and lacks thrust. You need the acidic bite of the canned juice in order to cut through the sweet coconut. The acid is the tip of the marlin spike, to use a naval metaphor.