Well maybe if you did a 3 week primary, then moved to a secondary for adding fresh pineapple pulp/juice, that way all of the active fermentation has completed and it wouldn't matter at that point if the yeast were killed off. Plus I dont see the acid from the pineapple killing off the yeast, since beer itself is so acidic. I could definitely be in the wrong though. I think I may give it a try in a cream ale or pale ale. I think it would be refreshing after a long day working in the yard this summer. Let me know how anyone elses turns out if any if you get a wild hair and decides to try this.