My take (and I could be wrong) is that dirt does not have the capillary potential of wicking fabrics because it is not composed of closely spaced fibers, but of discrete particles.
Also, you have to consider the mass of dirt as a whole, not just what is in the pipe. As per tapla's articles on GardenWeb and elsewhere regarding PWT (perched water table), the rest level is referenced to the lowest point of the contained soil. The water would rise to the PWT level measured from the sponge, and might not even reach the container, depending on the soil (e.g. average particle size).
Of course, the roots will find the water by growing down the pipe, but that won't support a large plant.
Worth a try, but I'll be surprised if it works well.
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