...and in addition to all that...
I think the question needs to be asked of an engineer at LRA for a definitive answer: "If the car sits on the weighbridge at GVM of 3240kg, can I then hitch up a trailer with a tow ball mass of 150kg and still be legal?"
It's a yes or no answer.
On another matter, as I said in the original post, we're about to head off on a long mostly highway trip to the Kimberley and back. I run Maxxis Bravo 980 light truck tyres on these trips, given that we're going to be loaded up pretty heavily, am I right in thinking that we'll need about 45psi in the tyres (cold) to handle the weight? (That's on the blacktop obviously, down to maybe 28psi on the Gibb.)