First, I am hugely sorry for ya Dan. I've been through something along these lines once long ago when a dealer wrecked my car and tried to hide it. It Fking sux when BIG man corp basically tells ya to crap in your hat and what are you going to do about it. I can’t believe they won’t even tear down to see if they can determine what the hell happened. Doesn’t BMW even want to know why an engine failed?
Second, I wonder if BMW has any idea how bad this is for brand loyalty? I mean everyone that hears about this will end of up hating BMW or liking BMW less. Have you thought about getting a social media movement going? Something like autoblog that gets the story out to the masses. I bet BMW NA might take more notice if a few hundred thousand enthusiasts heard about how they are treating you.
Third, I would video record the entire teardown process. If you do find a smoking gun then it will be paramount to have on tape. I wouldn't be surprised at all if something in the valvetrain let go and that's what destroyed cyl 6. I've heard of this happening a few times before on BMWs. Maybe a cam cap nut or one of the variable valve timing pieces (can't quite remember what it was). There is a chance the piece will still be in the engine or in the exhaust system.
Fourth, if possible please note the metallurgy of the pistons and rods. There's been so much conjecture out there it would be good to know for sure what's in our motors. Mike over at VAC could tell you exactly what to look for to determine.
Again I am SO sorry you have to go through this. It's absolute, complete BS. No two ways about it.