[percy]

Mike
I received an exemption certificate from ADR 4/00 (seat belts), which specifies that lap-only type belts are acceptable in the rear of my vehicle. As per the initial post, this was suggested by the ADR certifying engineer, and agreed to by the Engineering Section of the Western Australian licensing authorities.
I have a scan of my Engineer's Report, if you want a copy for reference.

I had a set of rear lap belts (very short !) custom made here in Perth, from an appropriately certified source.
When the car went over the pits for local licensing, I had to provide the exemption documentation and the ADR report, and explain the details behind the decision, but there was no drama.
So, after all this, it looks like I have a 4-seat cabrio, which can be safely used by two people in the front (with legs), and two people in the rear (without)

Bruce
Thanks for the detailed version of what had been explained to me by my local Porsche Master Tech. He actually showed me a 1995 car in the workshop, and I heed your comments about the massive centre buttress !