From the things I have seen in a auto body shop you might try a heat gun and slowly working in circles from the inside to the outside. The body shop fixies plastic bumpers that way. But I dont know how the heat would affect the airbag.
The heat gun shouldn't effect the air bag. I would worry that it would damage the outer cover. The heat would distort the vinyl cover. Your best bet is to remove the panel and work it from the inside. You should be able to apply the heat gun on the inside and slowly work out the shape. You can use your hand or a soft mallet with a soft cloth on the face side of the panel. Slowly apply heat in a circular motion and work the platic back into shape that way. GL.