one pusher, different bikes
Attached is my plan for a hitch that should work on various bikes including recumbents. I have had absolutely no problems with hitch placement at the seatpost and believe it to provide superior traction as a result of the tire to hitch angle. There is a tire loading action when the trailer pushes somewhat upward toward the hitch. That effect is lacking with a lower hitch. This has been noticed before. I believe Znsane Rider mentioned it here once.
More importantly, the high hitch position creates a pendulum effect that leans the trailer in line with the bike as they both lean around corners. This pendulum effect does what a much heavier, stronger, closer tolerance u-joint hitch must do when a lower attachment point is chosen. The lower hitch position results in a topheavy trailer that has been seen to prompt their builders to overbuild their hitches or switch to a 2 wheeled design. The pendulum effect must be achieved by arranging for more of the trailer's weight to be below a line from the hitch to the contact point between the tire and road. The entire wheel will be above that line, so the engine must be far enough below it to compensate. Stretching the engine to wheel distance helps with this as can be seen in the second attachment.
The higher seatpost hitch position allows the trailer to be somewhat longer and again, if the engine to wheel distance is stretched a little, the weight of the engine is close to the center of the trailer. If the trailer is not overbuilt, this allows enough flex to make a surprising difference in absorbing bumps & holes in the road. The same thing happens when a touring bicycle is built with a bit longer wheelbase and slightly relaxed frame & fork angles. The small increase in springiness greatly adds to comfort of the rider. Similarly, the first inch or two of 'suspension' in a push trailer is the most effective. Low weight on the trailer tire allows low tire pressure which gives all the added 'suspension" needed on my slightly springy trailer.
The single wheel design is preferable in weight, narrowness, simplicity, cost and cornering stability, especially with a seatpost hitch position that increases traction, allows a lighter, less precise 'u-joint' at the hitch and adds some 'suspension' effect.
My suggestion for a quickly removed throttle would be to use a brake lever with a clamp secured by a seatpost quick release skewer.
The use of wingnuts and quick release clamps could allow swapping the push trailer from bike to bike with no need for tools or parts remaining on the bikes.
Since I made the hitch drawing I have decided the best material would be HDPE (high density polyethylene) which can be found on eBay, in machine shop scrap bins or other more expensive sources.