AMENDED 2015-09-22
The library play-json-extra includes the play-json-variants strategy, but also the [play-json-extensions] strategy (flat string for case objects mixed with objects for case classes no extra $variant or $type unless needed). It also provides serializers and deserializers for macramé based enums.
Previous answer
There is now a library called play-json-variants which allows you to write :
implicit val format: Format[Foo] = Variants.format[Foo]
This will generate the corresponding formats automatically, it will also handle disambiguation of the following case by adding a $variant attribute (the equivalent of 0__ ‘s class attribute)
sealed trait Foo
case class Bar(x: Int) extends Foo
case class Baz(s: String) extends Foo
case class Bah(s: String) extends Foo
would generate
val bahJson = Json.obj("s" -> "hello", "$variant" -> "Bah") // This is a `Bah`
val bazJson = Json.obj("s" -> "bye", "$variant" -> "Baz") // This is a `Baz`
val barJson = Json.obj("x" -> "42", "$variant" -> "Bar") // And this is a `Bar`