If I were doing this myself, I would want to know the "isRetina" boolean on the serverside. You could include a common js script on each page that redirects /foo/bar to /retina/foo/bar for retina clients. You could then use URL rewriting to store the boolean in an environmental.
Take a look at this thread where I provide an example which does URL rewriting and storing values in the environment. You can then use the environmental value to perform custom serverside logic. http://tapestry.1045711.n5.nabble.com/Adding-a-dynamic-sub-directory-to-URL-td5714532.html
