To get back to the OPs question again, I don't think there would be a legal restriction to using a transferable machinegun for home defense. However, no shooting occurs in a vacuum. There could be very serious civil/social/political issues with using one.
While we are talking about Mas' articles, he wrote one recently about a old man who shot a coke fiend with a .410. The man had witnesses testifying that he shot in self defense. He still faced a difficult legal battle with the family of said coke fiend.
A machine gun can be a powerful weapon to defend your homestead and it could be a figurative weapon used to destroy you in the court room after the fact. I guess you won't know till you get there.
--For what it is worth, when I was a kid, my dad had a friend who had been a career Air Force man. He had a few transferable MGs. One of which was a Ruger AC556 that he kept in a closet. He referred to it as his "door gun." So, yes some people do keep registered machine guns for HD.
Stopping power: The power some guns have to make you stop what you are doing and get your wallet out.