As said I think it’s quite hard to find a reliable solution because you have to take care of so many things.
I go either for the way proposed by Danny van Kooten:
http://dannyvankooten.com/450/tinymce-wysiwyg-editor-in-wordpress-plugin/
add and open the editor in a thickbox or another way I’ve used once, using the widget “accessibility mode” for “advanced” stuff as seen in this screenr: http://www.screenr.com/sY5s
this example is buggy but the overall concept works fine, even if it’s quite funny that you have to use js to get and add the link to the corresponding (non-js fallback) widget-edit page 🙂
But on the other hand, on this seperate page you can actually do whatever you want.