Thanks for all your advice and comments!
Yes my farrier friend, from another continent, descibed to me all the methods of holding the foot of a smaller animal, about kneeling or crouching, but to be honest, I think at this point the relationship between mule and farrier is so long gone, that the farrier would probably rather quit than go down on his knees next to the mule, fearing having his head kicked in.
I really think the best recourse is for me to do it myself, and him to teach me/check it every five weeks.
On the weekend, I was seriously concerned about the length of toe he had on his rear hooves, and the angle is was throwing his pasterns into, so I decided to just trim the walls that are obviously overgrown, and sticking out far beyond the sole myself. I just couldnt wait any longer.
I thought it would be easy as the mule trusts me, but he was clearly so fed up with farriery of any kind, that it took a lot of groundwork for him to accept it. However, I can now kneel down next to him confidently and rasp the fronts, and managed to nip down/rasp the hinds. I don't feel as threatened by him kicking as the farrier does, as I obviously know him better, we trust each other more, and I know how to react when he does threaten. So I felt as safe as one possibly could, while still being aware.
So.... i'm sure I didnt do the best job, but i'm sure he is more comfortable nonetheless.
And afterwards, in a gesture to show he has forgiven me for all, as I sat down on a feedbin, quite exhausted, he came up to me and rested his chin on my shoulder and fell asleep.
I am so sure I am doing the right thing here, even if it will be a bit of a journey, and quite hard for me to master at first.
Thanks again for all your support.