Fig, I agree with you,
The Idea is not to tow the bus at 100 Kph, but to take it from place to place, at controlled speed.
The National Road Ordinance does allow for A-Frame towing, as long as the coupling is of Sound design, and capable of controlling the towed car.
Also the towed vehicle must be licensed and RWC, unless with trade plates, (got them)
Have been towing my DKW , and other cars this way for years, and still safer then mobi-jack's, dolly's or trailers.
As you said, you must ensure the vehicle, will allow itself to be towed, and therefore the A-Frame must be securely fixed to the car.
The idea to fix a A-frame with a belt, or even a temporary few bolts is looking for
The A-Frame that I will be making will have bolt-able fixtures bolted to both legs. (top & bottom)of the beam, This will form a solid, secure bolting point for the A-Frame. (if you use the bumper mounts of a VW aircooled, you may as well set fire to the car, and have pleasure in seeing it be destroyed, because it will rip out, and you will have an accident, and you will kill some one, as these were never designed to carry load.)
The A-frame will then be bolted with swivel, 100 mm under the beam, allow it to clear the bottom of the car by a nice margin, so as not to damage the car, and allow steering control.
All bolt's must be High Tensile bolts, well lubricated, with ball coupler and safety chain. Steering wheel will be fixed with bunggy cord to allow slight movement to follow tow vehicle, but to stop "tank-slap" steering.
Also a chevron with lights will be placed on back of bus when towing.
Just want to see if anybody has a better design then mine.
Herman