Rakefile

rule '.json' => '.yml' do |t|
  doc = YAML.load_file(t.source)
  doc = cfn_strings(doc)
  File.open(t.name, 'wb') { |f| f.write(JSON.pretty_generate(doc)) }
end

# finds any %{Ref: Thing} or %{Fn::Name: [Args]} inside strings,
# and turns the string into a Fn::Join with those objects interpolated
def cfn_strings(object)
  case object
  when Hash
    object.each_with_object({}) { |(k,v),h| h[k] = cfn_strings(v) }
  when Array
    object.map { |v| cfn_strings(v) }
  when String
    if object =~ /%{/
      parts = object.split(/(%{[^}]+})/)
      parts = parts.map { |p| p.start_with?('%{') ? YAML.load(p[2..-2]) : p }
      { "Fn::Join" => ["", parts] }
    else
      object
    end
  else
    object
  end
end