Ok I managed to get it through to App Store Connect! Had to replace the call to macdeployqt with Xcode's equivalents :( Got some helpful info from Apple, details on this thread if anyone's interested https://developer.apple.com/forums/thread/740606

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.

@YuChao-Hsu
That's Objective C++.
The code need to be in a *.mm file.