Hi!
We (@Zalando) are using {json:api} as a standard output JSON format. Since we are developing a lot of services with Scala, we developed a library to support that. The library is under Zalando organisation on GitHub.
It is still in kind of early stages but we already use it in production. There are some things missing but in general main use cases are covered. Feel free to contribute/star/watch/fork. Also check the issues to get familiar what we are up to next.
Ohh… It would be cool if we add Scala section under the server-libraries at the jsonapi homepage. 
Please give feedback!
Best