Here is a quick patch.  Not meant to be applied but meant for testing
purpose only.

So there is a html backend which doesn't add divs and there is a HTML
backend that adds divs. (The difference is between html and HTML, in
capslock).  

The `org-export-dispatch' uses HTML backend, while you can continue to
derive from html backend and add divs of your own.

Try this patch and let me know what you find.  Or you can circulate a
make-shift patch which improves the attached version to suit your needs.

I can formalize this patch once I get a go ahead from you.