I'm not going to waste my time looking for a law that isn't there...
Are we talking strictly commercial use, or personal? I was under the impression that we were talking about personal use, and there isn't a whole lot of info out there.
Interestingly enough, I was browsing Ramtrucks.com and noticed that the diesel 2500s got bumped up to 9600 pounds gross, which allows for about 2500 pounds of people/gear/payload, considerably better than the 3rd gen trucks. I also noticed that short bed 1-tons only get a 10k gross rating, but the long bed versions get a 12k+ gross weight rating. That's huge. Want a mega cab 1-ton dually to haul your family in comfort? You better not have a heavy trailer, because the payload is only 2500 pounds, and you can fit a lot of people and stuff in the cab of that truck. Heck, put 4 200-pound adults in there with no extra gear and now you better have less than 1700 pounds on the hitch/pin.
Unless someone can provide proof otherwise, I'm sticking to my guns. You cannot increase (as an individual) your gross combination weight. You may get a permit for overweight commercial transport, but you cannot alter the gcwr from the manufacturer.
Just because something isn't enforced does not mean it is legal. Take handicapped parking permits, for example - plenty of people buy off their doctors and get one when they don't qualify, but that doesn't make it legal. Same with removing emissions equipment - just because your local/state government doesn't do testing or inspections does not make it legal to remove or defeat the emissions systems on your truck.
I predict we'll see some new laws and/or case law hitting the books as joebob smith can now buy a pickup truck rated to tow over 20k. If he were doing it commercially, he would have to get a CDL.
I was blown away when I saw a pickup towing a large 5th wheel travel trailer towing a ski boat down the interstate on my way across the country. People can do that legally without any extra training or licensing requirements. Give it time, someone important/rich will get hurt and the laws will become more clear.