I was thinking of using RTF files for taking notes and thought it would be great if I could edit them directly in the preview pane of Opus.
Are there any plug-ins out there to do this?
I have considered using one of those note taking applications like MS OneNote, but why not just use the file system instead of creating a new notebook metaphor?
If you don't need RTF and plain-text editing would be enough then you might find the SourceCodeViewer plugin useful. It integrates a good text editor (SciTE) into the Opus viewer pane. (If you are using 32-bit Opus. There's no 64-bit version of the SourceCodeViewer plugin.)