But. It can be done.
Dig this fellow's tutorial. David Fesliyan explains how to do it. It's a bit, er, convoluted. But it makes a clean shadow pass. (Do yourself a favor and make sure "compositing" is left checked in the render panel or you will be as confused as I was for a substantial amount of time earlier today.)
|Ignore the unplugged lens distortion.|
There seem to be other ways to do shadow planes in Cycles but this method does not involve putting a garbage matte in over the ground plane. And it seems to work.
I say seems. I know not am.
I think at full resolution this render takes about two hours on my fastest machine. But it should make a right pretty composite so I'll let it go for the weekend and see what I've got.