Hi Louis, I think he meant the pressure of the weight of the grain against the doors is too big.
Standard containers have a couple more wrinkles (at least all the std 20 and 40 footers I have dealt with) . For one they have pretty good size vents at the corners that may be shielded but still have sizable holes for insects to enter and for grain to escape. Since there are subject to the elements they are also make the grain subject to moisture ingress and spoilage. Another issue could be the floor. To me the wood sometimes looks treated. I am not sure I would trust it with grain unless the mfg could ease my mind.
The loading capacity is surely limited by the weight and not so much how high you could fill it. Containers are customized all the time. I would not understand why you could not install smaller sliding gates (like on grain truck tailgates) on the bottom part of the std doors for unloading and something similar on the top for filling. When the container is empty enough you could open the std doors for removing the rest of the grain.