Substitute for pineapple juice in this recipe

I do believe your answer can be found in this totally over the top experimental comparison of two juicers. There's loads of info there, but I think the relevant part is when the author discusses the actual yields for each juicer. For pineapple, the author finds the yield to be between 71% - 76% of the total mass of the pineapple. So give it a weigh, and you should get about 3/4 that much juice. I assume the experiment skinned and cored the pineapple, so you'll need to account for that.

I'm assuming you're using a juicer to get your juice. Note that the juicers used in the above experiment were $400 and $500 respectively, so I assume they are pretty high end. You mileage may vary with a lower quality product.

And since you didn't ask for weight, I just tested weight to volume, 6 fluid oz of pineapple juice weighs 6.5 oz (so it's a bit denser than water). So one pound of pineapple should yield about 11 oz or just under a cup and a half.

I've never juiced a pineapple, but I have done a watermelon and I've had some success with a food processor and a cheese cloth. I'd remove the core of the pineapple, because I don't think there's much juice in it. Remove the skin as well. Then run the pineapple through the food processor and pour all of the contents into a cheese cloth over a bowl. Squeeze all the juice you can out of whatever's in the cheese cloth.

If you don't have a food processor, you can maybe use a cheese grater or potato masher instead.