Is there a way to schedule an upgrade of a ZD in advance, ex. cron job?

not from within a ZD as far as I am aware. Maybe if you have Flexmaster but not 100% sure if that can be scheduled.

Yes, there is a ZD CLI 'fw_upgrade' command, that you can issue at the time you choose to run it with CRON or similar.

I've performed a midnight TFTP upgrade for example with these commands, but didn't use any scripting tool.

ruckus#
ruckus#
ruckus# debug
You have all rights in this mode.
ruckus(debug)# fw_upgrade
------------------------------------------------------------------------

Name: fw_upgrade - Zone Director Firmware Upgrade Tool (Ver.1.2)
Synopsis 1: fw_upgrade <protocol>://<server ip|server name>/<path/image name> [-f]
Synopsis 2: fw_upgrade OPTIONS

Options:
        [-p http|ftp|tftp|kermit:Protocol for image transfer        ]
        [-s                     :Server ip address or name          ]
        [-n                     :Image name with path on the server ]
        [-f                     :Non-verbose mode
                     (may restore to factory configuration silently)]
        [-h                     :This help message                  ]

Examples:
        fw_upgrade ftp://<user>:<password>@&... ip>/<image file>
        fw_upgrade -p ftp -s <user>:<password>@<server ip> -n <image file>
------------------------------------------------------------------------
fw_upgrade
ruckus(debug)#

Thanks.. .that is very helpful.