I thought about this a bit more overnight. Another point or two if I may. A mechanical device used to raise a truck to dock height should be extra-heavy-duty strong if it will be used to support a significant amount of weight. In determining the weight, don't forget to include any forklifts that may be used to load the truck.
When truck bodies fail (the box), it is sometimes because they got beat up over time by forklifts. I don't mean the obvious where forklift drivers pucture body walls or drive through roll-up doors. I'm talking about the pounding a truck body undergoes again and again and again when forklifts enter the unit.
The metal plate found along the edge of the back door in most box trucks is sometimes called a crash plate. It has that name for a reason. When a heavy forklift with a heavy load leaves the dock and enters your truck, a tremendous amount of force is instantly introduced to (crashed onto) your truck. How that force is "grounded" or transfered from the forklift tires to the ground is a function of numerous factors.
If not properly grounded, the truck body is stressed beyond it's design limits and failure may occur instantly or over time. This is especially a concern in reefer trucks where you want the body to remain "tight" so as to retain insulation value over the years.
Think of the Brooklyn Bridge or any other bridge. Engineers have a number of ways of transfering the weight of the bridge itself and the objects on it to the ground. That's what all the girders or cables you see overhead are for.
So too with a truck. The the crash plate, rear frame, body frame, box floor, truck suspension, dolly legs, and more all come into play as weight-transfering devices when a forklift enters the truck. If the components themselves are stressed beyond their original tolerances, or if a modification changes the original geometry a failure could occur.
Electric or hydraulic dolly legs designed to raise a short truck to dock height may well be able to do just that. However, doing so would change the angle of the truck from what the engineers intended. They did not calculate loading forces with the truck raised to an angle. They calculated the forces with the truck sitting level.
Such a modification may work just fine at the visible level. I'd be concerned about the unseen forces that shoot through a truck every time a forklift enters. I'd also worry about the dolly leg device suddenly giving way under the weight of a forklift. That would cause the truck and any forklift driver that happens to be inside or halfway between the building and the truck to be in serious danger.
Finally, while the shipper mentioned may allow such a setup at his docks, I'm not sure how many others would.