Every rental car contract has the proviso in it that you can't take the car outside of regular roads or else the liability is yours. If you are so confident you are not going to smash the car on the race track, go ahead. Just be aware that in the event of any damage, you pay. BTW, most of the accidents there happen thru uninstructed fellow drivers.

BMW & Mini does a two day race track programme: http://www1.bmw.de/bmw-live/fahrertr...home/home.html