gistfile1.txt

userAgentLocale: Ember.computed(function() {
    return new Ember.RSVP.Promise((resolve, reject) => {
        Ember.$.getJSON([ENV.APP.baseUrlApi, 'util/user-agent-info'].join('/'), (data) => {
          resolve(data['accept-language']);
        }).fail(() => reject('...'));
      }
    )
  })),

  {{userAgentLocale}} shows [Object]
  
with help of https://github.com/kellyselden/ember-awesome-macros#promiseresolve

userAgentLocale: promise.object(Ember.computed(function() {
    return new Ember.RSVP.Promise((resolve, reject) => {
        Ember.$.getJSON([ENV.APP.baseUrlApi, 'util/user-agent-info'].join('/'), (data) => {
          resolve(data['accept-language']);
        }).fail(() => reject('...'));
      }
    )
  })),
  
{{userAgentLocale}} shows <(subclass of Ember.ObjectProxy):ember1696>
 
But 

dossier: Ember.computed(function() {
    return this.get('store').findRecord('dossier/dossier', 'b3dbf0ef-b2a1-5f31-aa7e-76d818d11e57');
  }),
  
{{dossier.id}} renders b3dbf0ef-b2a1-5f31-aa7e-76d818d11e57 so the idea must be possible...