Keith, I'm in the process of importing my '86 Cabrio and have run across the same problem.
The solution suggested to me was to use removable lap sash belts. This sounds a little unusual, but effectively it allows you to thread the sash part through the rear window zipper when the roof is down. This means the mechanical anchor point is still on the parcel shelf.
QuickfitSBS in the UK have supplied the belts for 95 odd pounds.
I've attached some images of how this works. Unfortunately I still think it fails ADR4D as they are removeable, and are static, but if you can get this signed off as a variance you'll end up with a safer outcome.
Rgds
Rob
