backbone.js sub routing

app.js

/* Pretend app setup stuff is here */

/* Kick off app */
jQuery(function($) {

  var Gallery = app.module("gallery");

  app.Router = Backbone.Router.extend({
    routes: {
       "add": "main_add"
    },
    initialize: function() {
      this.gallery = new Gallery.Router("gallery/");
    }
  });

  // Actually initialize
  new app.Router();

});

gallery.js

(function(Gallery) {

  var Helper = app.module("helper");

  Gallery.Router = Helper.SubRoute.extend({
    routes: {
      "": "list",
      "add": "add",
      "filter": "filter"
    },
    list: function() {
      this.navigate('add'); // -> redirects to app.Router.gallery.add
      this.navigate('/add'); // -> redirects to app.Router.main_add
    }
  });

})(app.module("gallery"));

helper.js

(function() {

  Backbone.Helper = Backbone.Helper || {};
  Backbone.Helper.SubRoute = Backbone.Router.extend({
    constructor: function(prefix) {
      var routes = {};

      // Prefix is optional, set to empty string if not passed
      this.prefix = prefix = prefix || "";

      // Allow for optionally omitting trailing /.  Since base routes do not
      // trigger with a trailing / this is actually kind of important =)
      if (prefix.substr(-1) != "/") {
        prefix = prefix + '/';
      }

      // Every route needs to be prefixed
      _.each(this.routes, function(callback, path) {
        if (path) {
          routes[prefix + path] = callback;
        } else {
          // If the path is "" just set to prefix, this is to comply
          // with how Backbone expects base paths to look gallery vs gallery/
          routes[prefix.substr(0, prefix.length-1)] = callback;
        }
      });

      // Must override with prefixed routes
      this.routes = routes;

      // Required to have Backbone set up routes
      Backbone.Router.prototype.constructor.call(this);
    },
    navigate: function(route, options) {
      if (route.substr(0,1) != '/' && route.indexOf(this.prefix.substr(0,this.prefix.length-1)) != 0) {
        route = this.prefix + route;
      }
      Backbone.Router.prototype.navigate.call(this, route, options);
    }
  });
})();

Ok, I may have figured it out. I added this code to the end of the constructor() function, right after the call to Router.prototype.

// grab the full URL
var hash = Backbone.history.getHash();

// check if there is already a part of the URL that this subview cares about...
var hashPart = hash.substr(prefix.length, hash.length);

// ...if so, trigger the subroute immediately.  this supports the case where 
// a user directly navigates to a URL with a subroute on the first page load.
if (hashPart && hashPart != "") {
    Backbone.history.loadUrl(prefix + hashPart);
}

With this code, I am able to navigate to http://example.org/#a/b/c, and have the "b" subroute trigger correctly. Please let me know if this sounds like a good approach.