Having something like the XBMC's adddon system sounds like a good plan, but wouldn't a package manager be a more logical choice? A package manager would, for example, make it much easier to handle dependencies between packages. There are
many package managers to choose from, but I personally think that the cross-platform, open source package manager
Zero Install would be perfect for this. One of the nice things of 0install is that packages can be managed both centralized and decentralized.
I wasn't aware there were such things readily avaliable; I assumed the arduino team would need to start from scratch, which is why I used XBMC as a model. But if someone else has done the leg work for you then that seems like a easiler, quicker and better solution.