You want to run your GWT hosted mode with the "-noserver" flag. By default the GWT hosted mode has its own little embedded webserver. This is really only useful for loading static resources.
What you really want is to run GWT hosted mode with just your GWT client code, so that your GWT hosted client is communicating with your regular app server. That's what the "-noserver" flag is -- it shuts down the embedded server and sets you up to load directly from your app server. You can Google for it in the GWT groups.
http://unrealities.com/seamgwt has a target for doing just that, though it's only with Tomcat and only on Seam 1.2 at the moment.