I too would love to see an Ubuntu One repository so I am going to include some possible references here for someone who might want to begin development:
First, there was an issue for including a dSpace plugin which also seemed to be REST based and may provide some insights. MDL-17025 contains some comments that may be helpful. https://github.com/enovation/Moodle-Repository-Dspace contains some code that was developed which may be of further help. http://docs.moodle.org/dev/Repository_API provides some information about what Moodle is looking for. Hopefully these links provide a starting place for someone wanting to dive in and have some fun. Peace - Anthony