respond-with

How to customize to_json response in Rails 3

I am using respond_with and everything is hooked up right to get data correctly. I want to customize the returned json, xml and foobar formats in a DRY way, but I cannot figure out how to do so using the limited :only and :include. These are great when the data is simple, but with complex finds, they fall short of what I want. Lets say...