You may wish to organise something with the others that posted in the Left 4 Dead thread, as I'll be away from tomorrow till the middle of next week, so relying on me will only result in nothing happening till then unfortunately.
20 Car Pile Up is far easier done on system link. Down 3, run off. The fourth will pick the other 3 up, then quickly hit the car/dumpster into them again. It'll take no more than 2/3 chapter run throughs.