There is (was?) a member by the nickname 'RickMcD' who works (worked?) at a Harley dealer just north of Los Angeles.
I just found out that it is quite easy to register vehicles in the state of Arizona (where I live) but the state is a little out of the way for someone coming in from out of the country. Your best bet is to contact Rick and start with him, and others in California, as California will be your closest and easiest way into the states from Australia.
I do know the local Harley dealer in Yuma, Arizona as the owner was my next door neighbor before he got the dealership (and the money that came with it, LOL), but again, it would probably be more of a hassle getting to Yuma than it is worth for you!