I'm only 5'9" and my primary skis are 186 cm = over six feet long. If I mounted skis on the rear of my Jeep XJ and didn't let them hang any lower than the bumper, then they would stick out waaay above the roof line. Maybe you could get away with it by mounting them diagonally, but I agree with the consensus so far: that's a messy and brutal location for your skis.
One trick I've learned about reducing the storage volume of a skis inside a vehicle (or roof top box for that matter) is to put a heavy duty rubber band around the binding brakes, so the brakes are held up above the ski base, in the same position they are stowed when you ski. Like this:
Load the skis into the empty trunk first, side-by-side and base down. Then you can stack the rest of your cargo on top of the skis and because the ski base is flush with the trunk, you hardly notice the thickness at all and simply have to stack the rest of your cargo around the bindings.
Now, you do need to fold down the back-seat, or have a truck bed, and unloading the skis is a bit of a pain because they're under all the rest of your cargo, but it takes up way less space than the more traditional approach of storing the skis base to base and interlocking the brakes. I've also seen this technique employed to "rack" the skis to the ceiling inside a car, which was pretty slick, but you'd have to make a bomb-proof rack to keep it safe.