I don't know how well this would work, I haven't worked with mules for quite some years now. Move their feet. I was watching a Clinton Anderson show the other day where he was helping a lady of with an aggressive horse. He actually ended taking an Australian stock whip in with him the first time to tell the horse he meant business.
I know mules and donkeys are different than horses, but I'd still make them move their feet. You need to have their respect, otherwise it's not safe. Which you know.