I don't think this would be possible. I might be wrong, but my understanding of the dropbox iPhone app, is that it is just accessing dropbox on the web, i.e. it's not creating a sync folder locally like on the mac version. The curremy version of the app is basically just a glorified webapp I think. And the favourites part of the app, is just used to download files from dropbox on the web to a local cache, which doesn't get synced.