HOw would gaining his trust by positive reinforcement not help themacpack??
So you would let yourself be associated with bad rather then good?
Sometimes, with aggressive and dominant horses the use of positive reinforcements encourage the behavior. Take my husband, for example. He has 20 horses he MUST ride EVERY DAY. 9/10 are soured up, and pin their ears at their neighbors. You're saying that opening the stall, bribing each one out with an apple is going to correct the problem?
Tempting with treats is 1) a bandaid solution when one is in a hurry, and 2) a sure-fire way to help encourage a horse's dominance over you.
In no way am I saying that you shouldn't give treats, or that it doesn't help some horses come around, because those are both positive things. But in this case, OP needs to assert her dominance in the stall, and let Sir Grumpy Britches deal with it.
And you know what? I would much rather be associated with a bad thing than get hurt. You can go back and erase most things that are "bad" that you're referring to, but you can't always take away an injury. And honestly, I'd much rather that horse have his feelings hurt than risk my health in any way, shape, or form.
The first thing I would suggest, OP, is to trade stalls with someone, if possible. At least for a short amount of time, to see if it's the stall he's protecting, or if he feels threatened in the enclosed space of the stall. (Which is very likely, he could feel cornered in there, or he could know that getting out means work and is avoiding you.) Whatever the case may be, he knows that he has you at his mercy when he's in the stall, and you need to correct that. If that means standing there, working him in and out of the stall for 10 hours, that's what you've got to do. You might even have someone help you, hold him outside the stall, you go in, and do not allow him in. Make sure he knows this is YOUR space, and he's just ALLOWED in it, and ONLY when you say so.
What are some of the other things you've tried with him?