Update!
I worked it out without using WebViews Midi.
It was as simple as calling a javascript function from onController.

function onController() { WebView1.callFunction("updateCC", { "cc":Message.getControllerNumber(), "value": Message.getControllerValue() }); }

@VeryHuman
Maybe you should have a look at the custom sample import demo project from Christoph:

https://github.com/christophhart/hise_tutorial