I'd be scared to put my hi-lift jack on my roof rack. I heard that if you hit a big ditch, the impact might break the mounts to the roof and that's all bad.
Cypilk, tater and dapit both use the demello clamps to put their hi-lifts on their roof rack. I use the garvin mounts to hold my hi lift on the stock roof rack. I've chased BZ around Mono Lake w/ the jack on my roof rack and it is still rock solid.
i would imagine you would have to hit that big ditch at a very high rate of speed or drop into it from very high above to break it. i would also imagine breaking the mounts would be the least of your concerns at that point.