After checking the documentation of UPrimitiveComponent, I found that bCastHiddenShadow is BlueprintReadOnly, which means that only getter for that variable is generated.
UPROPERTY(EditAnywhere, AdvancedDisplay, BlueprintReadOnly, Category=Lighting, meta=(EditCondition="CastShadow", DisplayName = "Hidden Shadow"))
uint32 bCastHiddenShadow:1;
So I can only confirm that you can only get this variable via Blueprint Editor and not set it. Sadly I don’t know workaround to its value in Blueprint Editor. One way to do it is to modify the source code, but I’m not sure if it’s the best way to solve the problem.
With regards.