I've spoken to a few mechanics and the blokes who do all my roadworthies; in terms of rust, if the car has any, it's unroadworthy. I guess it all depends how severe and how well hidden it is, also how much of a hardass your roadworthy guy will be.
I've also heard from a reliable source that the price of RWC have gone from $99 to $220 now. An attempt to prevent cars from being passed. They also take photos now.
I don't think this helps, but thought I'd let you know what I know.
Cheers, Jake.