Hydrating and feeding, as you said. You are right on. I would add that they need some exposure to direct sunlight every other day for an hour or so, as long as they don't overheat when that is happening. The UVB induces a skin reaction that ends up activating the skeletal and nervous up-take of calcium. Without UVB, their bodies will sacrifice bone calcium and give it to their nervous systems.
We don't really know how fast bones decay without UVB. But these iguanas need to be as sturdy as possible when let loose, so they can leap harmlessly from tree to tree as they establish their new territories. At first, they will have established iguanas chasing and biting them to drive them from their territories. It's got to be an extra nerve-wracking time for newly released ones.