As far as I know there is no possible way to execute an Extendscript via the C++ API.
In InDesign we can do so by providing the script as a string and fetching simple output results.

Our customers can customize our Plugins by providing their own script in our custom scripting language. Unfortunately our language does not cover all use cases and needs to be constantly expanded. With support for Extendscript/Javascript execution from the C++ API we could circumvent the problem and give the users a lot of flexibility and power.