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
A spring is then used to generate a 500g force for forward displacement.
After 3-months, you can see significant displacement displacement in the maxilla of the monkeys. All of the specimens had their wisdom teeth completely erupted.
Wearing a headgear on a full-time basis is not feasible for humans. This is why face pulling has to be complemented with tongue posture, to achieve a near-24h force on the maxilla.