I would still like to know how to make the DefaultText of an Editable Text widget editable in the Page editor, but overall my problem is actually solved.

In case someone else is interested, I was creating a Widget programmatically via the API, and adding it to an individual page, but couldn't access the content. You can access properties of the widget such as DefaultText and Container, but not the actual content.

The trick was that you need to add the widget to the page, save the page, then re-open the page. Now the widget is in the document, the content is writeable via the usual WebParts method.

eg.

// save previously created widget (based on EditableText webpart) back to page
CMSPortalManager.SaveTemplateChanges(docPageInfo, templateInstance, WidgetZoneTypeEnum.Editor, ViewModeEnum.Edit, tree);
// re-get page info and access widget content
insertedNode = DocumentHelper.GetDocument(insertedNode, tree);
myPageInfo = PageInfoProvider.GetPageInfo(insertedNode.DocumentGUID);
myPageInfo.EditableWebParts["Added_Widget_ID"] = "Content goes here";

Will.