So i'm not really sure how to go about it but basically if you look at this file https://github.com/intridea/grape/blob/master/lib/grape/entity.rb you will see this at 

line 305    # The serializable hash is the Entity's primary output. It is the transformed
    # hash for the given data model and is used as the basis for serialization to
    # JSON and other formats.
    #
    # @param runtime_options [Hash] Any options you pass in here will be known to the entity
    #   representation, this is where you can trigger things from conditional options
    #   etc.
    def serializable_hash(runtime_options = {})

So what i'm trying to do is convert the object coming into  def call to be a serialized hash and then generate the xml from that.  The reason I converted to json is that when looking at the other formatters I didn't see a clear way to get directly to a serializable hash.  Perhaps it would be better to make create some other object for dealing with this conversion process but as a first stab at it I put it directly in the formatter.

At the very least I think I added a test in the correct area so if you have a better idea of how to solve this then i'd love to see what you come up with.