My inclination would not be to bring them indoors - like all perennials, they need dormancy time. Try any way that you can to insulate the root ball, either by placing them directly in the soil in your garden (you can do this pot and all) or in a large container of soil, as you have suggested. I have water iris in a pot on my patio during the summer in a big Chinese bowl, but when fall comes, I sink the whole pot in the ground, and next spring they are ready to go again!