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Are there any code samples that show how to change Genesis facial expressions using the SDK?
No. But what are going to want to do is iterate across all the DzPropertys on a node, it's object, and all the modifiers(if the modifier is a morph and use its valueChannel's) to find the DzFloatProperty that you want to drive.
Porting the sample scripts located here to compiled C++ that uses the SDK rather than the script API should be fairly easy. The scripts are commented and show which classes are involved.