I doubt it could be done for a QQuickView embedded in a QWidget. But you can have a transparent QQuickView like:
QQuickView view;
view.setSurfaceType(QSurface::OpenGLSurface);
QSurfaceFormat format;
format.setAlphaBufferSize(8);
format.setRenderableType(QSurfaceFormat::OpenGL);
view.setFormat(format);
view.setColor(QColor(Qt::transparent));
view.setClearBeforeRendering(true);
view.setFlags(Qt::FramelessWindowHint);
view.setSource(QStringLiteral("./Test.qml"));
view.show();