just epoxy a magnet to the tip of the old plug.

Right.  I think it was assumed that someone suggested JB welding (not regular welding) a magnet to the inside end of the plug.  If the drain plug is steel, then you could just stick the aforementioned hard drive magnet to the outside of the drain plug, and it will make the whole thing behave like a magnet.

Chuck you are a genius

I would consider the possiblity that swarf would collect on the threads of the plug, this might damage the coresponding threads in the aluminium casing when you screw it out. I think if it was as simple a magnetising the whole plug the makers and pattern part people would have cottoned on to it by now. Also the steel the plug is made from may not be able to hold a magnetic charge for any long period of time.