In case anyone's interested, I've done a bit of digging and it seems the last specific firmware (that's readily available), where this should work is: v5.13(AAVF.7)C0. In v5.13(AAVF.9)b1, there's a line in the release notes that reads: "Support CLI interface".
FW available
here.
I can't be arsed to downgrade as the ZyXel FW isn't exactly bug free at the best of times, let alone when it's a few versions old. So I'll just go back to the Billion.
Paul