Thanks for your patience. I’ve merged this feature now:
To get at the VST3-internal preset data, you can pass an instance of ExtensionsVisitor to AudioPluginInstance::getExtensions. On the ExtensionsVisitor you can override visitVST3Client, and then call getPreset on the argument to that function.
Hopefully that all makes sense. Please let us know if you have any questions or run into any problems.