gistfile1.txt

$ curl -u admin:admin -F"jcr:mixinTypes=mix:referenceable" -Ftitle="Region 1" http://localhost:8888/test/regions/region1
$ curl -u admin:admin -F"jcr:mixinTypes=mix:referenceable" -Ftitle="Region 2" http://localhost:8888/test/regions/region2
$ curl -u admin:admin -F"jcr:mixinTypes=mix:referenceable" -Ftitle="Region 3" http://localhost:8888/test/regions/region3
$ curl -u admin:admin -F"jcr:mixinTypes=mix:referenceable" -Ftitle="Region 4" http://localhost:8888/test/regions/region4

$ curl -u admin:admin -F"jcr:mixinTypes=mix:referenceable" -Ftitle="User 1" http://localhost:8888/test/users/user1
$ curl -u admin:admin -F"jcr:mixinTypes=mix:referenceable" -Ftitle="User 2" http://localhost:8888/test/users/user2
$ curl -u admin:admin -F"jcr:mixinTypes=mix:referenceable" -Ftitle="User 3" http://localhost:8888/test/users/user3
$ curl -u admin:admin -F"jcr:mixinTypes=mix:referenceable" -Ftitle="User 4" http://localhost:8888/test/users/user4

$ curl -u admin:admin -F"users=/test/users/user1" -F"users=/test/users/user3" -F"users@TypeHint=Reference[]" http://localhost:8888/test/regions/region1
$ curl -u admin:admin -F"regions=/test/regions/region1" -F"regions@TypeHint=Reference[]" http://localhost:8888/test/users/user1
$ curl -u admin:admin -F"regions=/test/regions/region1" -F"regions@TypeHint=Reference[]" http://localhost:8888/test/users/user3

$ curl http://localhost:8888/test/regions/region1.json
is going to return
{
  "title":"Region 1",
  "jcr:uuid":"759d2b99-837b-4758-9e48-f4dff7958c1f",
  "jcr:mixinTypes":["mix:referenceable"],
  "jcr:primaryType":"nt:unstructured",
  "users":[
    "a6d6c596-aef5-470f-8212-fc8129aa5296",
    "b3342d99-75ff-4b70-bd3c-d3083d4b0298"
  ]
}

I'd like to see this:
{
  "title":"Region 1",
  "jcr:uuid":"759d2b99-837b-4758-9e48-f4dff7958c1f",
  "jcr:mixinTypes":["mix:referenceable"],
  "jcr:primaryType":"nt:unstructured",
  "users":[
    "/test/users/user1",
    "/test/users/user2"
  ]
}