I'd recommend trying to use a Filter instead of a default servlet whenever possible.
Other two possibilities:
Write a FileServlet yourself. You'll find plenty examples, it should just open the file by URL and write its contents into output stream. Then, use it to serve static file request.
Instantiate a FileServlet class used by Google App Engine and call service(request, response) on that FileServlet when you need to serve the static file at a given URL.
You can map /res/* to YourFileServlet or whatever to exclude it from DispatcherServlets' handling, or call it directly from DispatcherServlet.
And, I have to ask, what does Spring documentation say about this collision? I've never used it.