In a convertible like the Miata/Mx-5, a kilt is no problem. A motorcycle is also different than a bicycle (higher speeds, generally, and different mass between the knees).

When you drive the jeep with shorts, how much wind is in the knee area and which direction does it blow? If side to side or back to front, no problem. The only issue would be a strong blast from front to back.
In my car, the wind blows from the middle forward, disipates, and then across the lap to the doors. My only problem is a tendency for the apron to lift slightly (in American left-hand driving seat. A short wind blocker (about 3-4 inches) deals with the vast majority of it.
Because of the different external shape and interior design (wind accelerators, decelerators, deflecting surfaces and so on), a Jeep should be at least somewhat different. Also, the door heights will also affect it (as do the window heights on my car-> in all but the hardest rain, I can drive top down and windows up -above about 45- and stay PREFECTLY DRY, except for my forehead!)

Of course, I've heard of a chap who drives his Ariel Atom (basically a 4 wheeled spaceframe with minor body panels and a decent engine) wearing a kilt, so some just don't care.