Quote - Only thing I could think of is to make some kind of invisible shader that picks up the shadow, and apply that to a ground plane. Does that sound right? Is there such an invisible shader included anywhere, or is that something I'm going to have to make up on the fly?

You can use Carrara's standard shadow catcher for your plane's shader.  Then add a distant light with no more than 25% brightness and turn on its soft shadows in its Effects properties.  Works great without buying a plug-in.

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