Ally you beat me to it. The choker layer was just as you say, I had never heard of it even as thick as 4".
Tom, the point of the perched water table is not to keep the sand layer moist but rather to keep the bottom of the layer moist with a greater surface tension in the sand than in the gravel. Most of the water gets flushed through leaving just enough moisture in the interface between the layers to encourage the roots to dig for the water supply.
On the point of the varying depths in the rootzone mix, didn't the USGA build a wildly contouring green at the golf house in NJ with varying depths? I remember hearing that they were playing with the idea of suggesting this in the future. Seems like an impossiblity to create interesting greens this way. It would take some serious challenges for the shaper working towards an end result.