My grandmother kept them alive in Montgomery, Alabama by putting 1 ft deep of 'pine straw' on top. Don't know about other areas further north.
kevin

I grow marginally hardy tropicals in the Pacific Northwest by cutting plants to the ground and covering with remay (or some other type insulation), a layer of plastic, straw and top it off with a big plastic pot. It works! We lose more to rot than low temps.

I think you're right, the only thing to remember is the deeper you plant, the worse the drainage is likely to be, and its the combination of cold and wet that does for many plants, especially tubers, bulbs and corms.