Some rack middleware to add headers to asset pipeline.

development.rb

class AssetHeaders
  def initialize(app)
    @app = app
  end

  def call(env)
    request = Rack::Request.new(env)
    response = @app.call(env)
    if request.path =~ /^\/assets\//
      # there maybe a better way to add headers
      response[1]["Access-Control-Allow-Origin"] = "*"
    end
    response
  end
end

Railscasts::Application.configure do
  config.middleware.use "AssetHeaders"
  # ...
end