There are a number of specifications that address linking in JSON:

  * JSON-LD http://www.w3.org/TR/json-ld/
  * HAL https://tools.ietf.org/html/draft-kelly-json-hal-06
  * Siren https://github.com/kevinswiber/siren
  * Uber https://rawgit.com/uber-hypermedia/specification/master/uber-hypermedia.html

Totally agree on the points against making up a field / format. Using standards where you can is a great way to bring familiarity & common tooling into your APIs. But don't focus too much on XML vs. JSON, use what you need / want, you can find a standard to support it.