Okay, so it turns out Unicomp made a mistake and they do in fact have a replacement cable for this keyboard. As I needed to order some replacement keys as well, I have ordered from them to combine shipping to Australia. As an "apology" from them they gave me free shipping (which to Australia is no mean discount).
Thanks fohat.digs for the very kind offer of sending me one. Hopefully someone else who needs it will find it useful!