I'd think about cutting the rod close to the castings, center punching, and very carefully drilling out the rod remnants with successively larger sizes of sharp cobalt drill bits. Once the rod is almost completely drilled out with only small rings left, the right sized pin punch should finish it off without having to drill with the last size bit that matches the complete size of the rod. Since the ss is much harder than the surrounding aluminum, this will require a steady hand, and probably a good amount of experience using tools.
Grind the cut end of the rod flush and square, and very carefully position the center punch.
I don't think the castings will take too much beating on, or torque before yielding.
I remember doing this once a long time ago, but the guy ended up replacing the corner castings some time later, because they were pretty wallowed out anyway.