Okey so the workflow is:

1. ZfrRest creaets a ResourceModel here: https://github.com/zf-fr/zfr-rest/blob/master/src/ZfrRest/Mvc/CreateResourceModelListener.php#L66
2. This strategy https://github.com/zf-fr/zfr-rest/blob/master/src/ZfrRest/View/Strategy/ResourceStrategy.php#L66 which is automatically attached returns the ResourceRenderer (which is injected and fetched using the "ZfrRest\View\Renderer\ResourceRenderer" so you can easily provides yours.
3. And here is the "default" renderer: https://github.com/zf-fr/zfr-rest/blob/master/src/ZfrRest/View/Renderer/ResourceRenderer.php

I think it could be quite easy to bring different serialization here but for now, I really only json_encode: https://github.com/zf-fr/zfr-rest/blob/master/src/ZfrRest/View/Renderer/ResourceRenderer.php#L102