Thanks to theradioboy's tip, I've implemented emblems for the shared files and folders in the latest Dropbox Share Nautilus script (v0.2.3).

Dropbox Share now automatically adds emblems to the files you've shared (even though these files are not in your Dropbox folder) and removes them when you unshare the files. Please note that you need to unshare the files via the Dropbox Unshare script (it's also included in Dropbox Share package - all you have to do is select the files you want to unshare, right click them and under "Scripts" you'll find both "Dropbox Share" and "Dropbox Unshare") or else the emblems won't be removed!