Leslie, I've been considering something like rings too, mostly for newly divided rhizomes since they're most at risk for mulch washing onto the rhizomes. Although I wouldn't do this because I don't care for the appearance, I've thought about how country folks around here often use old tires as rings for raised beds. It would actually be a functional way to keep iris varieties separated and easy to maintain between plants.
The new bed I mentioned above is one where I just put cardboard over the sod in early spring and mulched heavily on top. When the irises arrived I just cultivated the spot, about 10" in diameter, mixed the mulch there with the cultivated soil, and planted the iris iris in the center. Once increases start growing those rhizomes are right on top of the mulch- they seem to really like growing in decomposing mulch.