Okay, I have tried the roll bar strategy and wound up driving with a greyhound in my lap every time we tilted to the passenger side. They are not the kind of dogs who like slanted surfaces.
Also, they would be squashed in a rollover.
The problem with harnesses is that they are designed for Rover to quietly sit in his seat while you take him to the park or vet. When spending several hours on the trail, they just have to move around some. With my rig, they can sit, stand and lay down on the seat, they can also get down on the floor. They just can't go out the door and in my lap. Believe me, those boney legs HURT when they decide they need to have some closeness with you.
All the harness straps and seat belts just get them all tangled up and then you have to stop and unwind them.