From my understanding, The race cars had to be derived from a production car and the lightweight shells and alum panels were the necessary basis of the race cars. The main competition were the Capris and 911, both much lighter cars. I'm sure others will chime in with more explicit info but suffice to say, BMW wouldn't have spent the money to develop the lightweight shell of the street CSL if they didn't absolutely need to.
BMW should have done what Lancia did for group B. Make a few hundred cars and put them in warehouse A. Take a really long lunch with officials on the way to warehouse B. Meanwhile, staff move all cars from warehouse A to warehouse B. Win!