I’ve done this in a relatively simple fashion, using Qt and Unreal (also using Qt and Unity, and other things) … basically we bring up the 3D world renderer (unity, unreal, others), and overlay a Qt app on top of it, that has an entirely transparent window. The world renderer and the Qt UI are both connected to a third process that would gate messages between them. All input to the Qt app that the Qt app did not interpret is passed through to the third process, which then passes it down to the world renderer.
I do wish that someone would come up with a direct integration of QML to Unreal, that’d be awesome. 