My gift to you this Valentine's Day. Rendering custom attachment fields in the new media manager via Backbone.

gistfile1.js

/**
 * The code below renders additional custom attachment fields into the
 * new media manager.
 *
 * I am assuming you are using your own custom media modal frame. By
 * extending the default Attachment.Details subview, we can append our
 * custom fields to the default fields listed.
 *
 * In the initialize function, we ensure that our changes are always up
 * to date by "listening" to our model's change event and updating the
 * view when the event occurs.
 *
 * In the render function, we ensure that the parent render function is
 * fired to display the default attachment fields. Once that is done,
 * we detach the current view, append in our own custom fields via a
 * custom script template (change "soliloquy-attachment-fields" to your
 * own script template ID) and pass our model's JSON representation so that
 * the "data" object can be localized to our instance.
 *
 * We then use this.model.fetch() to ensure that we have the most updated
 * data from the server and then re-render our updated view.
 *
 * The "return this;" line is the preferred convention in all render functions
 * in Backbone to maintain chainability.
 * @link http://backbonejs.org/#View-render
 *
 * @author Thomas Griffin
 */
wp.media.view.Attachment.Details = wp.media.view.Attachment.Details.extend({
    initialize: function(){
        // Always make sure that our content is up to date.
        this.model.on('change', this.render, this);
    },
    render: function(){
        // Ensure that the main attachment fields are rendered.
        wp.media.view.Attachment.prototype.render.apply(this, arguments);

        // Detach the views, append our custom fields, make sure that our data is fully updated and re-render the updated view.
        this.views.detach();
        this.$el.append(wp.media.template('soliloquy-attachment-fields')(this.model.toJSON()));
        this.model.fetch();
        this.views.render();

        // This is the preferred convention for all render functions.
        return this;
    }
});

soliloquy-attachment-fields template example: https://gist.github.com/thomasgriffin/4953100