I've read before that there are certain points where gear can be mag-locked on. I remember the list including the bonemark nub, shoulders (I suppose withers would be the more accurate term.) hips, and other major joints. Let me see if I can look it up...
Here we go, this is from the fansite here:
http://www.dragon-city.org/dragon_guide.htm"A dragon uses gear by magging the gear to its body. When a dragon mags gear, it creates a stable magnetic lock with the gear which fastens the gear solidly in place. This is similar to how the north and south poles of two magnets lock together when they come in contact. Mag-locks are exceptionally strong and the gear will stay put until the dragon wishes to release it or until the dragon's draconium energy is nearly drained.
Gear is usually magged to key points on a dragon's body called zap caps. These include the forehead, chest, shoulder region and other main joints, and are small bone protrusions where draconium energy emissions from the dragon are focused and can project more easily from its body. These zap caps emit visible magnetism in the form of electrical bolts of energy colored to the specific bone color of that dragon type."
Doesn't say much about where to put specific gear, but it narrows down your options.