Hercules makes some nice gravity-activated wall hangers with thick wooden backer panels. Like their instrument stands, the clasp mechanism opens and releases the instrument when the instrument is lifted.
If you can't attach the hanger directly to a stud, you'll want to make sure that the attachment method (screws, etc.) is reinforced behind the hanger so the weight of the instrument doesn't pull the hanger out of the wall.
And, if you live in a dry climate (like I do), you'll also want to make sure that you have a humidifier in the area where the instruments are hanging to keep the humidity at 40% (+/-).