Simple upload and download for certain. If by "sync" you mean a real time or file watching kind of thing, then definitely not that. I'd like as much as the API allows of rsync's delta-transfer algorithm to minimize data sent over the network and it's lqquick checkrq algorithm to quickly and intelligently decide what needs to be copied and what does not. Basically I want everything that rsync does (or everything that's possible given whatever API limitations there are).
I'm working on FUSE acd client as a perl-based application. I now able to partially download files (it is not covered in documentation, but i discovered the way though experiments), and now looking into way to partially modify/append files. Anyone knows the way?