Ah, I had a mare like that. It was frustrating. I used a neck strap with the lead running through the nylon halter EVERY TIME I tied her. And EVERY time I tied her it was to something very strong. No way was she going to break free. She would go for weeks at a time without a problem, and then decide she was going to pull. Random. I just didn't trust her not to pull, so I wanted to make sure that if today was the day I would be ready and she could not get away.
You'd think she would learn after the first dozen times, but she never did. She would pull so hard she'd end up on her butt, settle down and then a month later she'd do it again.
I would not tie her with only a rope halter. You need the support of a neck strap to keep her head in line with her neck or she can get hurt. I suspect the body strap does much the same, but I would also be worried about getting caught up in the lead below her neck. The neck strap I used was a 4 inch wide piece of nylon DOT strapping, quadruple stitched at the rings. You can buy such straps as well I believe.