Howard, what I have seen at local shops that fabricate intricate stairway systems, including the curved bottom steps, is that they use a plywood that has the same wood veneer as the solid risers. They kerf-cut the section that is to be bent, with kerf cuts spaced about every inch or so, and only about half-way through the thickness of the plywood, which seems to be about 3/4-inch plywood. possibly thicker. They steam the cut plywood in a chamber, and then carefully bend it around a jig-form to give the final shape and proportions. When the assembly has dried and the curve is set, they reinforce the curve using spacer boardsattached to the parallel portions of the bent plywood. In some cases, I have seen a plywood piece that is cut to have the inside dimension of the curved section and glued and nailed from the iside of the special riser. In any case, it calls for ingenuity and pre-planning of how to go about achieving the result you need. Good luck.