Stuart, I guess it sounds like the "onus of proof" in terms of a "true ICV" is clearly with the owner trying to register it. I would imagine that a series of photos of the car in progressive stages of build in a private garage, along with a bunch of receipts made out to a private owner, might be sufficient.
It also sounds like it would not matter how many "individuals" were involved in serially building the car - as long as there was some reasonable evidence that a single "individual" was responsible for its construction during the phase of his ownership (see above).
With regard to the purchase of an previously-registered interstate ICV, and its subsequent rego in NSW, it looks ok providing (as Trevor points out) that it is demonstrably a "true ICV". In my understanding, there is virtually no difference between the States in their interpretation of the ADR's, EXCEPT, emissions/engines. Here, I think that NSW is a bit more strict than other states. All now have a "rolling window" of acceptable engines, but NSW's is smaller than other states (3 vs 5 years, I think). So the only worry I would have about an interstate ICV is if its engine was one that met the rules in VIC/QLD/WA at the rego time, but would not have met the NSW rules at that time ?
If the RTA knocked such a car back, & you were a lawyer with plenty of spare time, I reckon that you could have some fun with them over Section 92 of the Constitution - Restraint Of Trade Between The States !!
Kind Regards,
Peter D.