<metadata>
   <![CDATA[{"xy":[50,50]}]]>
</metadata>

XmlElement el = ...
Element domel = el.getDomNode();
CDATASection cdata = domel.getDocument().createCDATASection( "..." );
domel.insertBefore( cdata, null );

import org.eclipse.sapphire.modeling.xml.XmlElement;
import org.eclipse.sapphire.modeling.xml.XmlValueBindingImpl;
import org.w3c.dom.Element;
import org.w3c.dom.Node;

public class CDataXmlValueBindingImpl extends XmlValueBindingImpl {

    @Override
    public String read() {
        return xml(false).getText();
    }

    @Override
    public void write(String value) {
        XmlElement el = xml(false);
        Element domel = el.getDomNode();
        this.removeAllChildren(domel);
        Node newNode = getNode(value, domel);
        domel.insertBefore(newNode, null);
    }

    private void removeAllChildren(Element domel) {
        while (domel.hasChildNodes()) {
            domel.removeChild(domel.getFirstChild());
        }
    }

    private Node getNode(String value, Element domel) {
        if (!value.isEmpty()) {
            return domel.getOwnerDocument().createCDATASection(value);
        }

        return domel.getOwnerDocument().createTextNode(value);
    }
}

...
@CustomXmlValueBinding(impl = CDataXmlValueBindingImpl.class)
ValueProperty PROP_SOURCE_CODE = new ValueProperty(TYPE, "SourceCode");
...