I can't say for 100%, but that doesn't sound like a normal material to use. Using steel into aluminum will cause corrosion faster than the steel will corrode by itself. Maybe some previous owner didn't know any better. Now I understand why you're having so much trouble. A blow torch to heat the plug up could help, too. For the heat to be effective, you have to get it red - a normal propane torch usually won't do it - but a MAP gas torch will. Remove to the topside access plate to vent the heat. Is there anything inside the bracket that would melt or catch fire?
Since it's steel, drill that little starter hole I mentioned.
Worst case, drill a bunch of holes so you pull the pieces of plug out.
I wouldn't just leave it - eventually it will corrode the aluminum, as well, to the point where it leaks and is not easily fixed.