The correction messages fall into two categories - the satellite clock/position errors, which are obviously useable whether the end user is in Japan or in Australia; and the ionosphere corrections. We can extend MSAS to provide the ionospheric corrections over Australia if we add a ground reference stations to the MSAS reference network.
I understand how the corrections are made and what they are correcting, my point was without the ground stations in Australia, that see the satellites that a receiver in Australia would see, a WAAS correction from Japan would not be technically possible.
However, I don't see any practical advantage in this approach over the extend-MSAS approach.
We should leverage on equipment available elsewhere to reduce costs, it would be pointless having a system which requires Australia specific avionics.