I know for normal CrossDOS Floppies you'd put your new default icon as def_PC0.info in env-archive: (and ENV: if you don't want to reboot).
You should be able to do the same for the zip drive, but the name will probably be different.
--edit--
also, if you're using OS3.9 you should be able to hide the DF0 icon in the Workbench prefs.
