If someone created an application that used application indicators, which are famously supported across multiple Linux desktops, would it benefit them on Windows or Mac OS?
Potentially, yes. They use DBus as a transport mechanism and this is available for Windows, for instance. So you'd write a Windows notification icon that rendered itself based on the DBus data and the indicator would appear as a normal Windows notification icon. The same is true for OS X.
The application indicator falls back to using a GtkStatusIcon that will work on other OSes. You won't get as nice of menu integration, like what you have on Ubuntu, but it should work. Now, all that comes with the warning that I don't have a copy of Windows to test it.
I wrote my own Python replacement for
appindicator when I was developing StackApplet. It runs on any platform with PyGTK installed (including Windows).