Bending exercises should be done in the saddle, IMO. You can feel a horses back and how they are flexing A LOT better that way, less of a risk of injury. As far as getting a horse to carry their head low, like BrokenWings said, I lunge my horse until he is tired and starts carrying his head at a naturally low level. Horses hold their heads at a fairly horizontal angle without pulling on them, if they are feeling nice, comfortable, and calm.... in my experience anyway.
Proud owner of 13yr old Paint Gelding, Bali! <3