There used to be a guy in Arkansas and he made some one wheel trailers that looked like a Thule cargo tube mounted on a small frame ABOVE the single wheel.
I never saw one in operation, and the website seems to be down now.
It looked to me like with the cargo above the wheel, that would reduce the vertical load on the hitch, but at the expense of a much higher center-of-gravity, and a higher twisting force on the hitch structure during turns.
It also looked like it would be subject to a speed-induced wobble on fast sweeping turns. I am constantly on the alert for any kind of trailer oscillation. I have not had any problems, and even in high speed sweeping turns on rough road surfaces, I have never felt any oscillations.
I CAN feel the trailer back there being moved around by the rough pavement, but have never had a wobble develop.
In a single-wheel design, in my opinion, the right way to reduce hitch load is by extending the overall length of the trailer, especially the distance between the cargo deck/box and the coupler. Not by placing the load higher above the wheel. If the trailer wheel is a smaller diameter, then that would help with getting the CoG down, and improve handling, but at the expense of trailer stability at moderate speeds.
On my trailer, the gyroscopic stability begins to kick in around 30 mph, and its fully stable by 40 mph. At speeds above that, the trailer is more or less self-balancing, and does not increase the riders workload at all.
I'm posting a pic (from another trailer thread) of another members trailer, which shows his bike hooked to a camper trailer. I'm guessing the tongue weight there is around 40 pounds. Maybe 50-60 if a fully loaded cooler is placed on the drawbar.
My trailer, in the second picture, when fully loaded, puts about 60 pounds on the hitch, but most of the time its NOT fully loaded and the hitch weight is closer to 45 pounds. I've measured this to be sure.
The last picture shows a similar design that would allow some cargo placement above the wheel, while keeping the CoG low. Its a picture I grabbed off the net last summer and cant remember where I found it.
It seems to show the trailer without a normal suspension, so I'm guessing its going to have problems with bouncing on rough roads.