Actually, the rules don’t say you can unload onto different islands, but they don’t say you can’t either. This is a designer clarification, and I am trying to get it added to the FAQ. This is a perfect example of why it’s important not to let the rules from one A&A game color your interpretation of the rules of another. We all tend to see things that aren’t there when we’re familiar with a similar subject.
It’s called Chaining and you can. if your Japan, and you have a ship in F, you can move ART and INF from Bougainville and Choiseul to NewGeorgia without moving the transport. Frankly the redeploy supply options will allow you to quickly drop land units off at Santa Isabel or Malita quickly. But if both are focusing on New Georgia, than there will be some big battles there.
Remember, Japan starts with a supply chain, the Allies don’t.