I'm not sure if there is a "correct" option (have an invisible object cast a shadow), but there is a slim chance this trickery might do the job...
What if you have your object out of camera view, located so it casts the shadow where you want it? You'd need really "sharp" shadow outlines to make up for the distance between the object and where the shadow lands.
Might be worth a try.
Here's a lame sketch, drawn in MSPaint with a mouse...
iClone 7... Character Creator... Substance Designer/Painter... Blender... Audacity...
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