Even if it would be possible it would be abuse of div tag and furthermore there would be the non-linear-edit problem with such a div.
I don't see how it would be an abuse. Keeping internal images in but not keeping external images out seems like a perfectly reasonable things to do, I just can't work out how to do it.
Linear editing is too restrictive. Where it is obvious when something should close, it's fine not to require it to be explicitly closed. Otherwise, you need to close it yourself. I'm sure you've seen the problems caused by forgetting the closing ''' after bold text - there is no way to handle emphasis linearly.
That said, this could be made to work linearly if the div tags were automatically added around sections with an appropriate class that could be referred to in CSS at the top. (Working on the assumption that a section finishes when the next section of the same level starts - a pretty safe assumption.)
But the most important thing is: Who would actually use it? Nobody.
Wikisyntax is not there for the sake of yet another makup language. Wiki syntax is meant for simple and concise markup. HTML is all but concise and simple markup. So my new suggest will maintain this simple and concise markup. I am not interested in finding the holy grail of true text markup.
While WikiSyntax is meant to be simple and concise, it is also meant to be independent of medium, something HTML was never intended to be. If we are going to achieve that goal, we have to sacrifice some simplicity.