I spoke with a guy who used steel drums for his mash tuns and boil pot. I think he used one for fermenting too, but I can't recall. I also don't remember what they were originally used for either. The problem is finding a burner that can deliver the BTU's to get that much wort to boil. Then you would have to have a fairly complicated wort cooling system, a high temp march pump, high temp food grade tubing, counter flow chiller, etc.
When you get into batches that big, those are small brewery batches and generally requires regular commercial brewing equipment.
It is also illegal to brew more than 100 gallons per person in the household with a limit of 200 gallons per year. Not that anyone cares..