well, i've seen 2 Rotus aluminum frames in person at Dennis' shop, so yes, they are aluminum. One frame (I believe this one) has larger cross section aluminum tubing to offer equivalent strength to the steel tube.
The other one, actually had different cross section tubes, some rectangular, and was re-engineered specifically for use of aluminum.
That's all I know.
I agree with Martin... you better be handy with welding aluminum to take this project on.