The official website says nothing about rider invitations, and the CDIs which I have helped run have never individually invited riders. I am still skeptical.
Also, considering the owner is in NZ and the show is in AUS, the flights over there would be expensive, regardless of if the horse is going as a competitor or as an exibitor. The show would NOT pay expenses for exibitors to fly over from NZ. So the post from the owner is moot. I have never heard of an CDI or an NF inviting competitors to a CDI3*.
ETA - I don't know why you are so adament about draft horses being built to do dressage!?! They have been bred for thousands of years to PULL not PUSH. Why all of a sudden magically would they be able to do the complete opposite of what thousands of years of breeding has developed?? It's not being a dressage snob, it's using the right tools for the job. It is unfair for a horse not built for a sport to make it perform and be uncomfortable and become lamed because it is not built for said sport. You wouldn't buy a smart car to pull a horse trailer, you wouldn't use a screwdriver to hammer in a nail and you wouldn't expect a horse built for pulling to be able to perform a high level of collection.
There are plenty of horses with huge hind ends that are rubbish at dressage. It's not about the size, it's about the angles, proportions and carrying power. I don't know if you subscribe to Dressage Today, but in the most recent magazine there is a "Physics of Collection" article which I highly suggest you read. Biomechanically, pulling strength does not equal carrying power and the ability to shift the pelvis under which is required for collection.