Where I disagree with you is over your assertion that it's physically impossible to have a tire 100 % inflated with N2 unless it is mounted in a 100 % N2 environment. Keep in mind that 100 % N2 may not even exist...
You are making my point -- if it doesn't exist, it's impossible to have it inside your tires.
... 99 % N2 is considered VERY pure. The reason I disagree with your statement, is because I've read about some N2 sellers which hook up a vacuum to the valve stem to purge all traces of air...
And, therein lies the proof. It is physically impossible to pull a 100% vacuum. Therefore, it is physically impossible to purge 100% of the air. Therefore, it is physically impossible to have 100% Nitrogen.
Once again, due to my (very) checkered past, I can back this up with actual experience. Before I retired, I operated a business that, among other things, dispensed LP Gas into cylinders. Because we also did LP gas installations, I went ahead and got Florida's highest LP Gas license rating, a "601" license, which required extensive education and exams, as well as practical experience. I operated that business for 10 years.
Part of the process of filling a new LP gas cylinder is purging as much air as possible and replacing it with as close to 100% propane as possible. There are two accepted ways to do it -- put a small amount of propane into the cylinder, which,being heavier than air, goes to the bottom of the cylinder and forces the air to the top, near the valve, then open the valve and let the propane push out the air. Doing this 5 or 6 times will result in about a 98% propane mix inside the cylinder. However, the advent of the OPD (Overfill Protection Device) valves on cylinders of 40# capacity and smaller made it very difficult to quickly purge in this method. Therefore, I installed a vacuum purge, similar to the one the tire dealers are using. I used a vacuum pump to remove as much air as possible, then closed that valve to hold the vacuum and opened a valve to introduce propane. The results were the same as manually purging -- about 98% or so propane.
If you think about it, it can't get any better -- the closer one comes to a full vacuum, the more likely it is that the vessel will collapse, unless it is specifically constructed to resist the pressure on the outside of the vessel caused by the vacuum. In other words, trying to pull a 100% vacuum on a tire would cause the tire to collapse.
Now, as far as recommending a different medium to fill a tire, I can tell you with absolute certainty what DOES work. On my New Holland tractor, I have purged much of the air and filled the tires with water. This provides the extra weight to help compensate for the weight in the front end loader and keeps the tractor from tipping over. I use a special valve that bleeds the air from the tire as the water is inserted (via a garden hose and the special valve). However, I only end up with about 80% or so water -- it's necessary to keep some air in the tires. I can use water because I'm in Florida; tractor owners in Northern climes use calcium chloride, antifreeze or -- believe it or not -- beet juice to keep their tires from freezing.