I would be very wary of using aluminium. When used in a regular bike it's fully triangulated so all the forces go through the tubes in compression or tension. With a Kyoto and with pretty much all recumbents you are using the frame as a beam with the forces being fed into the welds. Aluminium makes a poor beam with next to no give before failure. Also, unlike steel, when it does fail it will give no warning and fail completely whereas a failing steel frame would bend first or the weld would slowly peel away giving some warning of imminent catastrophe. Recumbents can and have been made from aluminium but it comes with substantial worries for a one-off builder, a fully loaded tandem probably more so than most designs.