Ghost 2003 can create a set of two boot floppies which include either USB1 or USB2 drivers.
The normal Ghost floppy is one disk, but adding USB support moves the actual Ghost application on to a second floppy.
<<Which is the earliest version of Windows that can access a USB HD from a floppy, or perhaps a special CD?>>
It has nothing to do with Windows, because Windows isn't running at the time you are booting to DOS. The hardware needs to support USB peripherals without Windows running, or load USB drivers in DOS.