There was another thread in the CSS Menu Writer forum that Ray responded to. Here is what he said...
Then in the resulting page open up the menu file and replace the simple text link in the sub menu with html content.
The menus we created are a lot like menu writer creates, we just replaced the text submenu with html that we generated by hand.
That will get you most of the way there, and of course you cannot edit the menu with menu writer once created.