I just realized, that it would be nice, if I could display Markdown documents rendered in the viewer pane. It would be as easy, as running the Markdown document through the Pandoc utility and display the result via the built in InternetExplorer plugin. But that would require DOpus to be aware of this special case for certain filetypes and to force the ViewerPane to use IE as plugin and have it load a different file, once it's ready.
Speaking of Pandoc, this would allow for many formats to be displayed as either plaintext (use text plugin) or HTML (IE plugin). And since an external converter would be used, one could even add awk and XSLT to the bunch.