I've just tested out the QQuickWidget class and it's working as I hoped. I can give each QQuickWidget it's own separate QMLEngine instance and the qml items inside each widget are not able to reach out into Qt widget land. There are still a few issues I'm looking into though regarding displaying one QQuickWidget over another. I've worked out how to make the top widget transparent but I'm still looking into event handling and how to stop the top QQuickWidget from eating events that should really be processed by the bottom QQuickWidget. I'm sure it's possible though. It's a shame that this isn't possible from QML itself. It'd be lovely to have a option you just pass to a Loader to say to sandbox the loaded item or something along those lines but this will work for now.

I found the solution. Not only can you call the HTML5 postMessage() API to pass messages back and form from/to the IFRAME to the parent document, but Qt's version of WebKit have lax security controls compared to Chrome and do allow you to do the following in the IFRAME's Javascript. The trick is the window.parent.[whatever] -- something that Chrome won't let you do with stuff accessed with file:// (local files). $(document).ready(function(){ if (window.parent.cpp) { var cpp = window.parent.cpp; // do something with C++ here through the cpp object } });