How to add (programatically) a custom icon in the Finder's sidebar favorite list?

When Dropbox (for example) is installed, it adds a “Favorite Item” in the Finder Sidebar Favorite Items List with his own icon and pointing to its custom folder. How do they do it ?

I’ve tried adding with PlistBuddy modifying the com.apple.sidebarlists.plist, but this is too limited, since I cannot add binary data, the bindings, TemplateSystemSelector, and other stuff needed in the array of the dictionary.

Maybe it’s a simpler way through Xcode coding…

Notice that with "Favorite Servers" is easier since it has just a Name and URL, and the icon is not customizable. So I'm not asking about Servers, but Favorites Items ;-)