What's needed is a highlight/styling widget that works with GT that can also be treated like an ordinary write stream. Then you subclass XMLPrettyPrintingWriterFormatter and override the formatting messages to style the stream differently at certain points (like before a start tag, after a start tag). Then change the GT "Source" pane XMLParser method to use "self writeXMLOn: (XMLWriter on: highlightStream formatter: highlightFormatter)".
The one drawback (besides performance) is that XMLWriter formatting support is just before/after tags, comments, and other markup elements. You can't add formatting within a start tag, which would be an issue if you want to style text in attr/xmlns values differently. I could fix this by adding more formatting messages to XMLWriter for before/after string values. Does anyone (Doru?) have more info on the GT aspects?
