This is a regression and works in 2.3-M1 for instance.

def rewrites: LiftRules.RewritePF = {
  case RewriteRequest(ParsePath("user" :: "logout" :: _, _, _, _), _, _) =>
    RewriteResponse("user" :: "byebye" :: Nil, Map("logout" -> Backend.logout().toString))
}

LiftRules.statefulRewrite.append(rewrites)

object Backend {
  object theSession extends SessionVar[Box[String]](Empty)

  def logout() =
     theSession.is match {
       case Full(session) =>
         theSession(Empty)//exception is thrown here
      case _ => false
  }
}

An exception is thrown when trying to set the SessionVar stating that no stateful session exists.