i can offer you a class. it'll filter all calls to /remoting resources and store the HttpServletRequest, Response and ServletContext for your request. from within your seam remoting funtion you can then use RenderToString.servletContext.get() for example to get the current servlet context.
if you got response request and servlet context, you can create your own facesContext and use that to render fragments into strings. my implementation doesnt work fully yet. but you can find a good sample if you search oreilly safari homepage for
Returning a JSF View Response to a Non-JSF Request

@Scope(APPLICATION)
@Name("org.jboss.seam.remoting.remoting")
@Install
@BypassInterceptors
public class RenderToString extends AbstractResource{
       
           @Override
           public String getResourcePath()
           {
              return "/remoting";
           }
          
           private AbstractResource original;
           public static final ThreadLocal<HttpServletRequest> httpServletRequest = new ThreadLocal<HttpServletRequest>();
           public static final ThreadLocal<HttpServletResponse> httpServletResponse = new ThreadLocal<HttpServletResponse>();
           public static final ThreadLocal<ServletContext> servletContext = new ThreadLocal<ServletContext>();
          
           @Override
           public void getResource(HttpServletRequest arg0, HttpServletResponse arg1) throws ServletException, IOException {
                   if(original == null) {
                           try {
                                   original = (AbstractResource) Class.forName("org.jboss.seam.remoting.Remoting").newInstance();
                        } catch (Exception e) {
                                e.printStackTrace();
                        }
                   }
                   original.setServletContext(getServletContext());
                   httpServletRequest.set(arg0);
                   httpServletResponse.set(arg1);
                   servletContext.set(getServletContext());
                   original.getResource(arg0, arg1);
           }   
       
}

I cannot possibly understand why you would want to use Seam Remoting to render portions of your page. I just got done editing chapter 12 of Seam in Action where I explain that messing with the rendered page without JSF's knowledge is generally a bad idea. If you don't like Ajax4jsf, try ICEFaces which automatically calculates which portions of the page should be rendered.

With that said, if you really do want to render something on the server, then you need to setup a FacesContext yourself, which is done inside of Seam for asynchronous method calls. Otherwise, you cannot render the Facelets templates.