While looking at the pictures from the Australian Nationals I noticed that the cleat and cheek block for the jib halyard are located at the center of the mast. On US boats the cleat and block are on the port side.

My question is this: Is it legal to move the cleat/block to the center of the mast from the port side in the US?

As far as I know moving the cleat/block is the only way to center the halyard while using the "aussie" halyard system.

Yes I believe it is perfectly fine to do as long as the cleat and block are the same hardware or the same type of hardware (horn cleat and cheek block).

This is standard in Europe since about 15 years. Yes, it is class legal but try to use the European cleat because that is a self cleating one and the halyard stays in it.
Advantage: The mast is no longer prebend to the side where the cleat is and you can adjust it while sailing downwind.
You should also add a fairlead about 50 cm down from the upper block, thus the battens pass without problems in front of the halyard without being caught.

Fritz