I've read the whole thread and thought I'd toss in a few lines, since I've had some experience with several horses like this.
First, a word of caution. We all know about a horse's instinct for survival, and that it includes flight. I won't bore you with all that. However, if a horse cannot escape, such as when you use gear the horse cannot break, once it figures out it cannot escape by pulling back, very often the horse will lunge forward, crashing into the hitch post, trailer, tree, wall, or whatever is in front of it. Make sure you NEVER GET IN FRONT OF A HORSE THAT IS PULLING BACK!!! Use a horse-tie knot the horse cannot pull loose, but will not bind and can be pulled loose with a simple tug. People get hurt by getting in front of a horse that is pulling back while trying to untie/unsnap a lead. Regardless, wait until the horse stops pulling back before releasing it.
I have a mare who normally stands quietly while tied, but a couple times has surprised me. Under different circumstances, she has pulled a couple times. She panicked and went to pieces. I knew she could not break my gear. I use a flat-braid nylon halter with buckles, rather than snaps, and a heavy-duty cargo parachute cord about 3/4" diameter with a breaking strength somewhere around 50,000lbs. I stood back and watched her pull. Each time she pulled, lunged forward, crashed into the post and fence, tree, etc, then pulled back again, finally ending up on her back with her neck fully stretched out against the rope, to the extent she could not get up. Once she stopped struggling, I pulled the knot loose and let her up. Each time she suffered a few minor scrapes and bruises.
The last time she decided to pull back, she pulled, settled back against the rope for a couple seconds, then seemed to remember something. She then relaxed and moved back up to the post. It was like I was watching her change her mind about pulling.
I don't necessarily recommend this method. It is dangerous to the horse and the handler, however, if a horse can break the gear, you will never teach them to overcome panic while tied.
The method I recommend is this: Take a long, strong lead, maybe 25'. Bring the horse to a post or tie rail and wrap the rope around it a full wrap, then walk out to near the end of the lead, to one side of the horse and hold the lead. You may have to have someone provide a stimulus for the horse to start pulling back. Some will just pull back anytime they are tied. As the horse pulls back, you hold tight and allow the lead to slip, however, you pull against the rope, such that when the horse gives, you pull it back to the tie rail. Make the horse work hard for every inch it gets, then pull it back in as soon as it give even a little. It eventually learns it gains nothing by pulling except to get tired, and eventually stops doing it. The horse learns he cannot break the gear, yet doesn't feel trapped into lunging against the post. It never actually panics, because it can actually pull some rope and is not tied solidly, yet it quickly tires of the game and decides it is not worth the trouble to pull.
I believe this is a safe method for gaining the same thing I gained with my mare. The reason I didn't use it on my horse was simply that I didn't realize my horse had this problem until it was happening. Each time was under different circumstances that caused the panic. The good thing with my mare is that she learns quickly and never seems to panic twice at the same thing. She has now learned that regardless of what panics her, she is better off panicking in place, rather than pulling back.
Oh, and don't use your best saddle while training this. My poor old saddle is now in dire need of repair.