I would think it'd be pretty stable. I've only used it with a rented 500 (might have been a 600) on a car window sill. I put a Kinesis beanbag on the sill, put a lowboy on top of that and then "dug" the feet into the beanbag (frame ultimately was flush with the top of the beanbag) so it wouldn't move forward/backward, left/right and then mounted the lense. It was pretty darn stable and was basically "unslideable." I was trying to see if I could get more "stickiness" than just resting the bare lense on a beanbag (and also experimenting with using it like that with a Wimberley and ballhead.) You could probably drive slowly with it that way although I wouldn't really want to.
