I forget the name of the bit, but it's a reverse threaded tapered "tap". You start by drilling a hole in the center of the rounded aluminum jacket to get your hole (This won't be hard to do seeing as there should be nothing behind it). Then use a rather large sized "Reverse threaded tapered tap". The "tapered tap" will continue to move in as you turn it to the left, until it gets to the point where it bites into the screw or nut or whatever, at that point, continuing to turn the tapered tap will only turn the nut, which will unscrew it from the stud.
Or, after screwing a little hole in the aluminum jacket, you sould have little bits of jacket "sticking out" see if you can use a pair of vice grips to pull that little fucker off exposing the hardened nut underneath (I doublt the nut underneath is rounded)