JSONAPI serialize nested hasMany relationships

We're using JSONAPI for this project, but for [reasons] we can't handle its recommended relationship structure in the API, so we're serving and expecting them as nested objects instead, with the following format:

{
  "data":{
    "type":"video",
    "id":"55532284a1f9f909b0d11d73",

    "attributes":{
      "title":"Test",

      "transcriptions":{
        "type": "embedded",

        "data":[
          {
            "type":"transcription",
            "id":"203dee25-4431-42d1-a0ba-b26ea6938e75",

            "attributes":{
              "transcriptText":"Some transcription text here. And another sentence after it.",

              "cuepoints":{
                "type":"embedded",

                "data":[
                  {
                    "type":"cuepoint",
                    "id":"bb6b0434-bdc4-43e4-8010-66bdef5c432a",

                    "attributes":{
                      "text":"Some transcription text here."
                    }
                  },
                  {
                    "type":"cuepoint",
                    "id":"b663ee00-0ebc-4cf4-96fc-04d904bc1baf",

                    "attributes":{
                      "text":"And another sentence after it."
                    }
                  }
                ]
              }
            }
          }
        ]
      }
    }
  }
}

I have the following model structure:

// models/video
export default DS.Model.extend({
  transcriptions: DS.hasMany('transcription')
)};

// models/transcription
export default DS.Model.extend({
  video: DS.belongsTo('video'),
  cuepoints: DS.hasMany('cuepoint')
});

// models/cuepoint
export default DS.Model.extend({
  transcription: DS.belongsTo('transcription')
);

Now, what we want to do is save a video record, and have it serialize the transcriptions and cuepoints it contains. I have the following serializer, and it works absolutely fine for embedding a transcription into a video ie. one level, but I need it to then embed the cuepoints into that too.

export default DS.JSONAPISerializer.extend({
    serializeHasMany: function(record, json, relationship) {
      var hasManyRecords, key;
          key = relationship.key;
          hasManyRecords = Ember.get(record, key);

      if (hasManyRecords) {
        json.attributes[key] = {};

        hasManyRecords.forEach(function(item) {
          json.attributes[key].data = json.attributes[key].data || [];

          json.attributes[key].data.push({
            attributes: item._attributes,
            id: item.get('id'),
            type: item.get('type')
          });
        });
      } else {
        this._super(record, json, relationship);
      }
    }
  });

Inspecting the record, json and relationship properties in the serializeHasMany method, I can't see anything about the nested relationships, so not even sure I'm using the right method.

Any ideas where I'm going wrong with this?