Your rotor idea will work if you design a follower that keeps the tube feeding onto the curve of the bender at a consistent height. You could also use the rotor set up in a press with consistent followers on each side to ensure uniformity. I'm sure you are aware that square or rectangular tubing will distort along the sides because of the top and bottom parallels bending on different radius. If your die has immovable sides that are the width of your material the distortion will be a nice trough inward on both sides. If you die is wider than the material the distortion will be outward and the material will flatten. You might try the filling the tube with sand (cheap mandrel) trick....not sure that will work or not, it depends on how compressible your sand (media in the void) is. I have no idea of how to make a real mandrel bender for square stock but I would bet someone does!
Looking at your cad....have you considered chatting with the Millers at Epocs? The unit could be made by cad laser cutting the 4 sides of the tube and welding them together!
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