Doug I would think iwagumi's are a subset of Zen Gardening, especially since I found out from
Roy Deki's Iwagumi article, the word Iwagumi means "rock formations". In Iwagumi setups, the idea is have rocks gracefully control and give purpose to the overall aquascape.
It happens that actual Zen Rock Gardens have zero plants, and are focused primarly on the rocks resting in sand. The rocks, sand and other componets will be reflective of nature i.e the sand is the ocean, and the rocks are the islands.
-John N.