I see your point.
The only easy way I see to accomplish what you are looking for is allowing the user to use placeholders for <div> ids in the template.
For example, the user can write the body of the email as follows:
having the template replacing the placeholder %content_1%, %content_2%, ... with the content of each <div>.
This way, though, the user would be forced to edit the HTML code of the body of each email.
I don't know if this is close enough to what you are looking for.