This feature is really required for many integration scenarios. In IE control it's possible to overwrite "window.external" interface with your functionality. It's possible to define your interface with method(s)/property(is). they would be visible in JavaScript. As result, JavaScript developers can call "external" functionality like normal JS calls... marshaling/mapping done automatically... for You, it just call of your method.
I hope Oracle would be able to extend WebEngine implementation and provide "similar" way to define/call Java methods from JavaScript.

With Best Regards, Nick

P.S.: Not sure if sub-task is correct way to add comments ;)