It's JavaScript, so the code will be visible. It will probably be minified though, so not very readable.
I think you'd need quite a few controls, I think. If you could get hold of the basic synthesis code, you could probably work out a method to control the parameters of the model that made more sense in the context of controlling it via MIDI CCs. Simply having a knob for every parameter of the model probably won't make much sense.
In text-to-speech systems, these kind of models are usually fed parameter streams made from analysed natural speech.
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