There is stuff at pollulu in the xxµA range. Keep the max current in mind, though.
You could add a switch to switch it on manually.
Both of these support an enable/disable pin:
Perhaps there is a possibility to do that kind of automatically, e.g. switch it off when device is unplugged (no current over the usb port), and restart with a pushbutton.
Is it a safe assumption that all devices will tie shield and ground together? Then one could use that a a switch. Connect shield on your side with 10kOhm to 5V. Then it will be high when nothing is plugged in, and low when a device is plugged in which ties shield to ground. Would probably have to invert that signal afterwards. No idea if that is something one would like to do or if it is a really stupid idea, though. I would not do it without further assurance.