Just saw your post - looks like we posted about the same time with roughly the same ideas except yours is more complete and works!
Are you thinking of forwarding the Spring MVC request to a Surf webscript to populate a portion of the page or a rss feed or something? I think you'd probably want to just go via a URL that Surf services so that you get all of the Surf processing. If you tried to just have a Spring MVC View Resolver go to Surf's view layer direct it would miss out things like the Surf interceptors. One of these populates the context object on which lots of the webscript stuff depends.
My View Resolver suggestion was for going the other way - from Surf to a Spring JSP but I'm not sure it would work... especially given the other thread about using JSP.