I have done a Responsive Site design for http://www.sherwinanderson.com/ -- It uses multiple MW navs on several pages.
Basically I set the key element styles for each element based on the media query (Phone, Tablet, or Desktop). You can see how the elements flow when changing the browser width. I did this in the main css file for the site -- the menu css files are unaltered.
I still have some tweaks to optimize, and it's not real elegant -- but it works for this application.
Developing a true "responsive design" CSS Menu Writer will be pretty challenging -- I've found that there's lots of "tweak and check" in doing RD, and getting the wizard to properly manage all the previews and stuff is going to be complex. . . .