Anything but maybe a running martingale is dangerous on the trail, in my opinion. A trail horse's job is to take care of himself. In order to do this, he needs freedom of balance. Horses use their heads and necks to balance going up and down hills. A horse may need to lower his head and inspect footing before proceeding.
Branches and vines may snag on your equipment, he may get a leg through it while we eats or lowers his head, etc. An even worse distaster would be your horse tripping with a GM on.
He would be unable to regain his balance, go down, thrash, still be unable to right himself, and hurt himself.
If you need a martingale, use a running. You can give him as much freedom with a running as needed by letting out your reins.
Also, have you been doing one rein stops when he "runs through" snaffle bits? Or any bit, really. Using two reins gives them something to brace against and run through. It's a lot harder to run away when you're only using one rein for control. One reins stops and lateral flexion may really help that head tossing problem. ;) He's also find that it's a lot harder work to be one-reined stopped every time he goes faster than a walker than to try to gait off with you. He may just choose to walk after a few consistant sessions.