I'm really glad to say that the unpack200 code has been completed on Harmony! Sian and the rest of the team who have been working on it are to be congratulated. I had done some of the initial work, but they really took it over the edge to make it happen.
It should be possible for those systems who don't have pack200 natively installed to be able to use this to download and unpack code. Hopefully, it still runs against Java 1.4, which means that it will be able to be bundled and used in previous systems, as well as those where there is no native pack200 support available.