I had the exact same problem with an Arab mare that came to me as a rescue. She'd kick like a demon if anyone even touched her hind end.
I solved it quickly and safely by using a very soft cotton rope. I simply looped it around her fetlock (not with an actual loop, just both ends in my hand with the end of it cradling her fetlock. I would increase the pressure and firmly ask "foot". She kicked something fierce, but with only a soft rope, no damage was done and she was able to easily be free of it as soon as I released the pressure. I just let her kick against the tension in the rope, talking to her and soothing her until she realized nothing bad was happening.
Just a few sessions of this and I was able to graduate to actually picking up her back foot and progressing with working on them in typical fashion. Within a month she was standing beautifully for the farrier.
Actually restraining a rescue horse who's only acting out of fear is just plain dumb. She may get her back feet again, but it's going to take twice as long to train her properly. I understand her feet are an issue, but with just some gentle sessions she can easily be schooled in the proper direction and enjoy a lifetime of good feet with no force.
Best of luck!