Similar situation, I wanted to auto-backup to Backblaze.
From what I can see, the update functionality relies only on a Y/N confirmation, which can take a default value, whereas the export functionality uses input prompts, which simply aborts if given the --no-prompt option (see here).
I think the prompt function’s code would have to be modified to abort only when not given default arguments, as well as take inputs from command line arguments.
EDIT: I found this: Running “ghost backup” automated in shell script with Expect // manually works, crontab doesn’t I think this is a good alternative