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!
Because there is no way to refresh Nautilus by directly calling it via command line, I've used a "hackish" way to do it: the script will send "F5" to the window (just as if you've pressed it) which should instantly make the file emblems show up. If for some reason the emblems are not updated automatically, simply press F5.
Here is a video demoing the new emblems feature in Nautilus Share 0.2.3:
Install Dropbox Share in Ubuntu:
sudo add-apt-repository ppa:nilarimogard/webupd8 sudo apt-get update sudo apt-get install dropbox-share
Dropbox Share is now available on Launchpad so you can easily submit bugs: https://launchpad.net/dropbox-share