Thanks, anda_skoa!
That's exactly what I did - and it works! I have a rather complex model with several lists in it and it was not easy to get it done, but here are a few details for others with the same problem. This is not a compileable example (too big), it may have been shortened too much and may contain simplification mistakes - but it might perhaps be able to shorten the solution finding process of others anyway.

I need several buttons, each with several data fields on it. The latter are defined here:
Qt Code: 
  1. class DataFieldContent: public QObject
  2. {
  3.     Q_OBJECT
  4.     Q_PROPERTY(QString text READ text WRITE setText NOTIFY textChanged)
  5.     Q_PROPERTY(QColor color READ color WRITE setColor NOTIFY colorChanged)
  6. signals:
  7.     void textChanged();
  8.     void colorChanged();
  9. public:
  10.     [...]
  11.  
  12.     //getters
  13.     QString text();
  14.     QColor color();
  15.  
  16.     //setters
  17.     void setText(QString text);
  18.     void setColor(QColor color);
  19. };
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All my data fields are organized in a C++ class "Content":
Qt Code: 
  1. class Content: public QObject
  2. {
  3.     Q_OBJECT
  4.     Q_PROPERTY(QQmlListProperty<DataFieldContent>dataFieldContents  READ dataFieldContents NOTIFY dataFieldContentsChanged)
  5.  
  6. signals:
  7.     void dataFieldContentsChanged();
  8. public:
  9. 	[...]
  10. 	QQmlListProperty<DataFieldContent> dataFieldContents() { return 
  11. 		QQmlListProperty<DataFieldContent> (this,
  12. 											0,
  13. 											&appendDataFieldContent,
  14. 											&dataFieldContentsCount,
  15. 											&dataFieldContentAt,
  16. 											&dataFieldContentsClear); 
  17. 	}
  18.  
  19.     static void appendDataFieldContent(QQmlListProperty<DataFieldContent> *list, DataFieldContent *p);
  20.     static int dataFieldContentsCount(QQmlListProperty<DataFieldContent> *list);
  21.     static DataFieldContent* dataFieldContentAt(QQmlListProperty<DataFieldContent> *list, int i);
  22.     static void dataFieldContentsClear(QQmlListProperty<DataFieldContent> *list);
  23.  
  24. protected:
  25.     QList<DataFieldContent*>m_dataFieldContents;
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As I need a bunch of those Buttons, there is also a GroupContent, basically a List of Contents (Note that QList does not derive from QObject, you can't "just use" it).

Qt Code: 
  1. class GroupContent: public QObject
  2. {
  3.     Q_OBJECT
  4.     Q_PROPERTY(QQmlListProperty<Content> contents READ contents NOTIFY contentsChanged)
  5.  
  6. signals:
  7.     void contentsChanged();
  8. public slots:
  9.     void contentsChangedSlot();
  10.  
  11. public:
  12. 	[...]
  13.     QQmlListProperty <Content> contents();
  14.  
  15.     static void appendContent(QQmlListProperty<Content> *list, Content *p);
  16.     static int contentsCount(QQmlListProperty<Content>*list);
  17.     static Content* contentAt(QQmlListProperty<Content> *list, int i);
  18.     static void contentsClear(QQmlListProperty<Content> *list);
  19.  
  20.     void addContent(Content* p) { m_Contents.append(p);}
  21.  
  22.  
  23. protected:
  24.     QList<Content*> m_Contents;
  25. };
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On the QML side we need a display: MyButton.qml:

Qt Code: 
  1. Item {
  2.     id: ButtonBasis
  3.     property var dataFields;
  4.  
  5. 	Grid {
  6. 			[...]
  7. 			Repeater {
  8. 				id: dataFieldRepeater
  9. 				model: ButtonBasis.dataFields
  10. 				Text {
  11. 					text: dataFieldRepeater.model[index].text
  12. 					color: dataFieldRepeater.model[index].color
  13. 					[...]
  14.  
  15. 				}
  16. 			}
  17. 	}
  18. }
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And all that is linked together in main.qml:

Qt Code: 
  1. Repeater {
  2.         id: contentRepeater
  3.         model: groupContent.contents
  4.         delegate: MyButton {
  5.             dataFields: contentRepeater.model[index].dataFieldContents
  6. 			[...]
  7.         }
  8.     }
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Don't forget to register your classes to QML (e.g. in main.cpp) by:

Qt Code: 
  1. qmlRegisterType<DataFieldContent>("de.yourdomain.whatsoever.whatsoever",2,0,"DataFieldContent");
  2. qmlRegisterType<Content>("de.yourdomain.whatsoever.whatsoever",2,0,"Content");
  3. qmlRegisterType<GroupContent>("de.yourdomain.whatsoever.whatsoever",2,0,"GroupContent");
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