Scala Play WebSocket - One Actor, multiple message types

What I want to do is basically 1:1 this: Scala Play Websocket - use one out actor to send both: Array[Byte] and String messages

Sadly the API has changed a lot since 2.4 (I am on 2.6.0-M4 right now).

What I tried (does not compile for obvious reasons):

WebSocket.accept[WSMessage, WSMessage]
    {
      request =>
        ActorFlow.actorRef
        {
          out => WebSocketActor.props(out)
        }
    }


  sealed trait WSMessage
  case class StringMessage(s: String) extends WSMessage
  case class BinaryMessage(a: Array[Byte]) extends WSMessage
  case class JsonMessage(js: JsValue) extends WSMessage

  object MyMessageFlowTransformer
  {
    implicit val WSMessageFlowTransformer: MessageFlowTransformer[WSMessage, WSMessage] =
    {
      new MessageFlowTransformer[WSMessage, WSMessage]
      {
        def transform(flow: Flow[WSMessage, WSMessage, _]) =
        {
          AkkaStreams.bypassWith[Message, WSMessage, Message](Flow[Message] collect
          {
            case StringMessage(s) => Left(s)
            case BinaryMessage(b) => Left(b)
            case JsonMessage(j) => Left(j)
            case _ => Right(CloseMessage(Some(CloseCodes.Unacceptable)))
          })(flow map WSMessage.apply)
        }
      }
    }
  }

I am somewhat lost. play.api.http.websocket.Message is a sealed trait, and probably for a good reason...