I successfully used richhtml4eclipse as the WYSIWYG editor for an internal software project - You get a dialog with native widgets for style buttons, etc, and a plain looking window for the HTML view.
I also applied some post-processing to the output with the Java version of HTML Tidy, but that was more for version-control consistency reasons.
This is a native JFace container with an embedded Browser widget running a page with tiny_mce embedded.
It uses an older version of tiny_mce for it's internal HTML editor, and connects to it via an approach I've seen used elsewhere - passing events out to the native Java app via status bar changes, and accepting input via JavaScript calls on the Browser widget.
