I just started a project that will require 6-12 months of waiting, but I am posting it now to encourage others to give it a try in parallel with me. The recipe is for Coquiña or Cachaça de Coco that Charles H. Baker, Jr. wrote about in
The South American Gentleman's Companion. Baker acquired this recipe from the Furna de Onça Restaurant in Rio de Janeiro, Brazil, and is for coconut-aged rum! He presented it as follows:
Take husked coconut, bore out 1 of the "eyes" in the "monkey's face" you'll find at 1 end. Pour-out water. Refill nut with any white rum like new Cuban or Key West aguardiente, or inexpensive white Puerto Rican or Cuban rum - NOT brown or Jamaica. Cork tightly with a soft-wood peg driven-into the shell firmly. Bury in ground, dig-up after 6 month to a year. Drink the stuff neat, out of small glass.
The process will add a ruddy amber color and a "most unusual and pleasant taste." Moreover, it will make the spirit more mellow the longer it stays in its tropical cask. Baker did give the option of cachaça later in the text.

Earlier today, I bought 3 coconuts at Whole Foods. The best part of the experience was when the cashier dropped one from about a foot up. She apologized and said that she did not want to break it before I got it home. I told her that it could be replaced, and beside, it would take a lot more than that to crack a coconut. She then asked how I was going to open them and recommended a hammer. When I finally told her that I was going to drill a hole in each of them and fill them with rum, she shot me a dirty look. The stunned and judgmental expression quickly shifted to her ignoring what I just said and re-recommending the use of a hammer to open the nut.
For a method, I did end up using a hammer albeit later in the process. I used a 1/2 inch drill bit and made a hole in one of the eyes in each coconut. The wider the eye, the easier the bit went in for there was less tough, fibrous matter in the way. I then filled two with Seleta Cachaça and one with J.Wray & Nephew Rum. I did not realize until afterward that the directions specifically stated not to use Jamaican rum, but I now interpret that to mean dark aged and not white overproof Jamaican rum (which could stand a bit of mellowing). I sealed the holes with pieces of whittled tree branch and the recommended hammer. Instead of burying them, they are being stored in the kitchen for safe keeping.
I am curious to see if the rum will become coconut-flavored à la unsweetened Malibu or fat-washed with coconut oils. Only time will tell since Baker was rather vague about the effect.