Pinpoint push notification template with raw JSON using message helper

I'm creating push notification template with raw JSON.
However, including message template helper in JSON attribute causes a following error while sending message.

PERMANENT_FAILURE Invalid template: Mismatched start/end tags

This is the message template I've created.

{
  "APNSMessage": {
    "aps": {
      "alert": {
        "title": "{{#eq Demographic.Locale 'en_US'}}English Title{{else}}Other{{/eq}}",
        "body": "This is notification body."
      }
    },
    "some-custom-attribute": "foo"
  }
}

Message template helper in standard message works as expected, but I prefer using raw JSON in order to add custom attribute to push payload.
Is it possible or any alternatives?