Couldn't you create and share a virtual file system with FUSE?
There are ways to intercept writes without root and send them to another machine. Eg you could use LD_PRELOAD. But that's exactly the kind of pain in the ass that I was hoping a project named FFmpeg-over-IP would automate.
User space limits potential security impacts, and a restricted VFS could be used to prevent clients from accessing anything that they shouldn't.
(Although I'm not even pretending to know whether or not this is a remotely good idea - my guess is that it isn't, but I'd like to know just how bad an idea it actually is.)