Cut rhubarb in 1" pieces; add water to cover. Cook until soft, about 10 minutes. Drain through cheesecloth bag or sugar sack. Measure—should be 3 qts. juice.
Dissolve sugar in 2 c. water; cook 10 minutes to make a syrup.
Add lemon, pineapple and rhubarb juices. Pour over chunk of ice in punch bowl.