You can either use CSS to force text wrapping, or use the richtextviewer in plain text mode with an onclick microflow.
Used the CSS solution which now works fine: Created a class in the layout.css (called 'DMLink') for the dynamic microflow trigger widget:
.DMLink .mendixButtonlink .mendixButtoncaption{ text-decoration: none; color:#333; white-space: normal;
And to keep it compatible for IE7:
.djquirks .DMLink .mendixButtonlink .mendixButton_content { white-space: normal;