Once you use a brine with an acidic base like pineapple juice, you run the risk of breaking down the proteins in the meat (mush as you say) if you leave it in too long. You may actually have a marinade depending on the salt content. In either case, brine or marinade, I wouldn't deviate from the recommended usage. 48 hrs may indeed get you where you don't want to go. Stay with an hr/lb if you use the pineapple recipe.
You can tell by the recipe if it's a brine or marinade. If there's a lot of salt, like 3/4-1 cup kosher/gallon of liguid, to encourage the osmosis affect, you have a brine. If the recipe calls for minimum salt just for flavoring, then you've got a marinade. In either case, if pineapple is the main source of liguid, go with the hr/lb.