There is no way to change the QTextDocument instance of a QQuickTextEdit. QQuickTextDocument provides only read access to the QTextDocument instance. That is a custom QTextDocument subclass (QQuickTextDocumentWithImageResources) and therefore cannot be made interchangeable - a plain QTextDocument wouldn't work correctly with QQuickTextEdit.
=> QTextDocument should provide means for QQuickTextEdit to do the resource handling without subclassing QTextDocument.
This is a rather big restriction for applications with multiple documents, because each document requires its own instance of QQuickTextEdit. This is not only clumsy but also very inefficient because each document, visible or not, has a view attached and thus does heavy text layouting when the content changes in the background.