I do horse rescue, so we've had a few with hind feet problems.
One was in training with Charles Wilhelm.... he had issues like you describe, and had some scaring on the front of both hind cannon bones. No idea if they were related, but he was on a slaughter lot when rescued, a day before shipping to Canada...
anyway, I got him to let ME handle his feet, but the point was that anyone that was working with him needed to, and be safe.
Charles' method worked for him in a day... and was simple.
He used a buggy whip (the length that is like 4 ft long, with a one foot loose ropie thing at the end) and did not attempt to touch the leg at first with 'his' hand, just with the whip.
He desensitized him with it first, which was easy as Charles uses a whip to do all kinds of extension of his arm in the exercises on the ground and in the saddle...
then he tapped on the fetlock... asking him to lift the foot...
when he did, and held it for a second, he got a 'good boy'
when he swatted or kicked... he got a sharp 'no' and a hard rap with the whip.
no emotion in it, and no lasting thoughts on 'good' or 'bad'...
he kept asking... tap tap, and rewarded the lift and hold, asking for longer and longer...
and gave him a negative on the kicking...
in a five minute span of time (for each side) this guy was lifting his foot on a tap, and holding it for however long til he was patted and allowed to set it down.
then, when Charles or the farrier gave it a try...
amazing but he just lifted it nicely, and held it for them no problem...
never had to do it again.
Now, on to another of ours, who came with a HUGE proudflesh on his left hind fetlock, the size of a softball... said tissue was cut off by first vet, who said the horse 'tried to kick his lights out' when he 'got anywhere near the rear of the horse'... not good. Horse had to be fully sedated to even LOOK at the hind foot while in quarantine (he was also a feedlot rescue... sad. someone dumped him to the killer due to a cut on his leg) I got him... huge open wound on his entire back of the rear fetlock... needing twice daily dressing changes... and medication... LOL...
With this boy, what worked was talking to him, and when he lifted it, holding ONLY the hoof... and not the fetlock (which hurt!) and letting him take his time to relax it and then I could do the dressing. I had to let him know only a few times that kicking out was unacceptable... past that, I lifted, and held, and did that a few times, then had farrier ask... but was specific to him to grab the hoof... that way the horse knew what we were doing.
Many times the farrier will grap the fetlock to position the foot... and that must have hurt still.
This horse is still with us, and three years later fully sound and competing in gymkhana with my daughter.... and still is worried about the hinds, but if asked nicely, and allowed to let his anxiety pass... will let anyone pick up his feet now.
I hope you can find a way thru this... you do need to let them know it is unacceptable, and let them know what IS acceptable, but beyond that, it is desensitizing...