I don't believe so - nothing short of customized enclosures by a shop.
Only a few companies sell custom enclosures, I can only think of JLAudio and MTX. MTX does not make anything for BMW. I'm guessing because, just like the reason all bmw mods are so expensive, is the volume is not there. A low percentage of people in the world own a BMW, and an even lower percentage of those people choose to upgrade the car.
Go with the Stealths if you want this type of setup. Unless you go w/ a really good audio shop, AND get a great deal w/ them - having them design a customized enclosure to replace the rearspeakers would be more than the 400 JL is asking for IMHO.