Hi Rogier,
I don't think this is possible currently. Feel free to file an issue at https://github.com/mendix/RestServices/issues.
I'm not sure what a nice solution would be, because this problem is more generic, json keys can contain arbitrarily string data, but probably adding support for serializing RestServices. Primitive objects can solve the major of these issues. (You could then instantiate a Primitive object, and the serializer should use the name of the primitive instead of the reference). Would that be a solution?