Thank you all so much for responding. She's not on any serious meds yet and won't be for the next 3 months - we'll be travelling and the doc is wary of trying new stuff while we're away.
First - the video usually works but didn't yesterday. During an anger fit, all reason is (seems) gone. Can't do anything logical.
Second - yes, there's no way she can actually jump since all the override buttons are on so I'm not worried about that.
Third - Stopping the car is logical and right and we did but in the future I'm worried since we live in the desert and we can't sit in a car for longer than ten minutes without the engine and her fits usually last an hour. Also, her little brother - excuse me while I burst into tears - I'm scared for her little brother.

This situation might never arise again but if it does, I need a plan. I'm driving them today - if it happens, I might have to call 911. Jo - thanks for all those links, I'll look into them. If I had a safety lock, that might work. Or a barrier to keep her in the back row away from her brother and me. Or I could do as I did yesterday and require that my husband come with us everywhere we go ... not!

Sorry this is such a long post.