In theory it might be possible to test this experimentally. Rather than genetically modify them from the start, synthesize the protein in question and try to g

In theory it might be possible to test this experimentally. Rather than genetically modify them from the start, synthesize the protein in question and try to get it into cell cultures. This should make it possible to get an idea of what will happen in that situation, e.g. will it kill the cells or cause anything obviously wrong. After that test it on some animals with similar Vitamin C problems. Shouldn't be impossible to test this, after the animals test it in adults to find out. etc.