It is definitely possible to do the wall balls without cranking your neck back; your peripherals should be able to track the ball at the top just fine.  The trick is getting people to actually think about their neck position instead of just defaulting to looking at the ball/target straight on.  Sometimes moving a bit further from the wall allows enough space for better position, as well.  The main thing is to be aware and focus hard on maintaining that neutral spine. 

Compound issue: if the athlete has trouble squatting in an upright position, this movement does become virtually impossible.  It might be good to warm up with some wall squats (arms overhead, toes to the wall) to get the good positioning.  If you are teaching a class, you might want to leave the wall balls for a few weeks in, after you get participants up to par on that vertical squat position.   You could also start on a lower target and work up to the proper height as squat position improves.