I found a very interesting study showing that maxillary displacement is possible in adults. Researchers applied 500g of force to the maxilla of adult monkeys through a teeth anchored appliance for 3 months. They then determined the maxillary displacement with the help of bjork implants. This is a very reliable technique, and it proves that facial bone movement is definitely possible in adults.
This is the Vitallium splint that was used. As you can see on the picture, it is tooth-anchored.
The monkeys were immobilized in a headgear for a duration of 80-91 days