The point of this thread is moot.
NFA weapons are only accessible to people who have the money to spend and can get through the complex paperwork process. Usually they are dealers and have FFLs. There are a very limited number of people in the US who have these weapons. Those who have these weapons do not leave them lieing around the house or out at night, but generally stored and locked down. Most of these people that if the police took their NFA weapons then they probably would not get it back...at all. They also believe they are so expensive that leaving them out makes them more stealable.
So no one is going to use their NFA weapon for HD most likely unless it just so happens to be out of their sacred locked up stash.
Not really moot. I would imagine that most full-auto weapons are very expensive. However, a SBS or SBR is not. $200.00 patience and diligence is all that is required for the stamp. You can obtain a 590 Mossberg for less than $500.00. With some basic tools and metal working knowledge you can own a SBS.
People who can afford 20 grand for a Thompson, can also afford a really good attorney.
I would also imagine anyone who is into collecting firearms to the point where a 20K Thompson is part of that collection. That person is going to have a designated HD gun as well as a plan for using it. I would think this person would also have a home security system of some sort to protect a valuable gun collection.
You never know though. On one hand I think it would be crazy to have a weapon of that value for HD. You take every worldly possession I own and pile it in the yard. I couldn't buy a Thompson with it, no where near it. I also don't have a possession I would kill for. I have a HD for the same reason most of us here do. To protect my family. So on that note, if I had one, and if I had to, I would use it to protect what is most important to me.