Maybe I missed something basic here...
Out where I live in New York, you see every single truck with a hitch ball. That makes taking a TT seem like an every day occurrence. I must say, however, that finding a 5th wheel hitch is *rare*. I couldn't even get one installed locally! At the campground where we have a seasonal site, there's maybe only 10 of us out of 100+ trailers.
The actual percentage of 5th wheel units actually going missing/stolen would be a wonderful number to get from an insurance company. Determine how much of a risk theft is in this (as Curt puts it) "specialized towing" situation. :p
Before you consider anti-personnel devices, perhaps it's better to just attach a lawyer on a choker collar to the kingpin. Seems to be legal in all 50 states, provided you supply water and minimal food.