Unfortunately I am not smart enough for the fancy node tricks. :(
My dumb-painter approaches are:

1. an IBL light set low with a color complementary to the main shadowing light.

2. a very dark complementary color set on the ambient channel of everything in the scene to fake an ambient IBL... gradients can be used here also.

3. a separate shadow only render that is composited to a no shadow render and colorized in Photoshop.