If its useful to anyone, I've written a node.js script to do a two way sync with a single dropbox folder. I'm using it to sync a folder on my BeagleBone so it should work ok for rpi too.
It uses the dropbox delta api to get a list of changes and can also detect which files have changed locally since it was last run. Unfortunately it has to be run manually to do a sync at the moment, but it's better than nothing.
I did try to use inotify to watch files, but I get an error that too many files are open, so I'm not sure if this approach is viable.