I can imagine if you knew the horse was going to roll, it wouldn't be too hard to keep them on their feet. Just kick them and get them moving and they won't try anything. A horse has only rolled with me once. The thing is, I had no clue he was going to roll. Here's what happened:
I had just mounted and was adjusting my stirrups. Dakota was just walking around and I wasn't paying attention to what he was doing. He stopped and put his head down and so I gave him more rein, assuming he was just going to scratch his nose on his leg. I was staring down doing my stirrups and I felt some sort of a weird motion... I can't really describe what if felt like but I didn't think at first he was going down. Then all of a sudden I realized he was on his knees, and my feet were almost touching the ground!
It's amazing how smooth it is. I didn't even realize what was happened. Suddenly I knew I had to get off immediately. I just kinda slid my feet out and stood up, and he was between my legs for a second before I just walked away. It was weird. Then he just started rolling! I was just kinda standing there looking at him. The funny thing was that he wasn't doing it to crush me... I would have been killed if I hadn't gotten off, but Dakota seemed to be waiting for me to get off before he rolled. I guess he figured he couldn't roll while I was sitting there!