Maybe we don't need to modify anything (signaturewise)? Maybe this is enough?

Posts.scala

// ...

  @Signature(parameters=Array("post_type", "subdomain", "group", "title", "body", "author"))
  def create_post(post_type: String, subdomain: String, group: String, title:String = null, body:String, author:String) = wrap {
    // not sure if this is efficient, but you get the idea
    val CreatePost = post_type match {
      case "short_post" => CreateShortPost(GroupRef(subdomain, group), body, UserRef(author))
      case "long_post" => CreateLongPost(GroupRef(subdomain, group), title, body, UserRef(author))
      // ... case else invalid
    }
    driver.runReadWrite(CreatePost)
  }

// ...

posts_controller.rb

# ...

  def create
    post_type = params[:post][:type]

    params[:post][:post_type] = post_type
    params[:post]['subdomain'] = @subdomain
    params[:post]['author'] = @author

    @post = Post.create_post(params[:post])
    @errors = { errors: 'Unknown post type.' } unless @post.valid?
 
   # ...
  end

# ...