actually... the hardest part would be the pump, the actuator and the safety stuff. the rear knuckles will accept movable tie rods easy... change the upper strut mounts of course.

yes a durability issue might have come in to play when developing them

If you look closely at the rear suspension, and you know what you're looking for, you can see that it was in the planning.

Several cars have had 4-wheel steering all the way back to the early 80'*, and currently in some GM trucks.

I remember a friend of mine hada 4 whl steer Prelude....That thing was WAY too much fun......But I have looked back there before......I can see a lot of the provisions that could create rear steer.