QQuickRenderControl: rendering to an existing QRhi?

QQmlEngine engine;
QQuickRenderControl renderControl;
QQuickWindow window{&renderControl};
QRhi* rhi = ...;
QRhiRenderTarget* tgt = ...;

// ... init ...

window.setGraphicsDevice(QQuickGraphicsDevice::fromRhi(rhi));
window.setRenderTarget(QQuickRenderTarget::fromRhiRenderTarget(tgt)); 

renderControl.initialize();

// ... update resources ...
// How do I get the Qt Quick updates into my QRhiResourceUpdateBatch?
QRhiResourceUpdateBatch& res = ...;
renderControl.polishItems();

 // ... render ...
QRhiCommandBuffer& cb = ...;
// How do I pass the command buffer to the render control?

renderControl.beginFrame();

// next call causes: "QQuickRenderControl cannot be used with QRhi when no QRhiCommandBuffer is provided" which makes sense, but I cannot find where I can pass the QRhiCB
renderControl.sync(); 

renderControl.render();
renderControl.endFrame();