After the last post I've had time to have a bit more of a think about the whole problem (another WTF moment, if you will). An idea crystallised after looking at Pats non working NZR couplers (or coupling bar), as well as thinking about the British tension lock system, and my own first forays into NZR modeling where pocket money didn't extend to fancy couplers. Hence the following was born.
A brass crass brace was soldered on and then the assembly's were attached to the wagons for testing.
The coupler will push and pull happily round a 15" radius curve, and probably a bit tighter. the slight reverse curve in the coupling hook means that when there is a load applied the couplers are pulled down towards each other rather than square where there is a possibility that one will lift away from the other.
As a prototype I think it has a fair bit of promise for a non working coupler which makes as least as much sense as anything else we use. There needs to be a bit of work done on the shape of the neck, but its not a huge problem to solve.. Rakes of wagon can have the coupler of your choice at each end. Its not quite as flexible as regards to wagon facings, but you can't have everything.
UPDATE: Darryl has provided an excellent sketch of what I'm talking about.