> I need to be able to asynchronously get data from all kinds of places before I can even think about calling React.renderToString

You can define middlewares, as you would do server-side with expressjs:

    router.route({
      method: 'GET',
      path: '/users/:userId',
      handler: function (ctx) {
        // load user async
        getUser(ctx.params.userId, function (err, user) {
          ctx.user = user;
          ctx.next(); // exec next middleware
        }); 
      }
    });

There is a `demo` folder with a detailed example. The main entry point for the client is here https://github.com/gcanti/tom/blob/master/demo/client.js#L21