I'm confused. You can use literally every webassist extension and every other tool available in Dreamweaver to edit your email body now that it is opened as a separate page. We didn't used to have a tool for editing an email body, now you have all of the tools Dreamweaver has to offer.
If you want to edit the email body, just open the Email Server Behavior and note the body file. Then you can open that file and edit it freely using all of the tools at your disposal inside Dreamweaver.