Passing unsaved record to Ember.js route

Inside an application we allow users to create new records, related to an existing record. To achieve this, we use actions something like this:

createUser() {
  var route = this;

  var model = this.store.createRecord('user', {
    client: route.modelFor('client'),
  });

  route.transitionTo('user.update', model);
 },

The user.update route renders a user-form component, using the model that was passed in the transition. The same route is also used to update existing users.

The issue with this approach is as follows; when refreshing the page, the page errors because the route fails to find the respective record when querying the store (at this point, the URL is /users/null/update). Ideally I'd pass the client (or client.id) argument in the URL so that:

1. The page can be reloaded without issue.
2. The client associated with the user is set correctly.

How can I achieve this in Ember.js? I know that this can easily be done using nested routes (by nesting the user.update route inside a client route), but this doesn't make sense visually.