This is theoretical... ref msdn and so ... being on a background thread makes things interesting :)
var request = new HttpRequest("/", "http://example.com", ""); //hopefully you can hardcode this or pull from config?
var response = new HttpResponse(new StringWriter());
var httpContext = new HttpContext(request, response);
var httpContextBase = new HttpContextWrapper(httpContext);
var routeData = new RouteData();
var requestContext = new RequestContext(httpContextBase, routeData);
var urlHelper = new UrlHelper(requestContext);
var url = urlHelper.Action("ActionName", "ControllerName");