Seems Half Pipe is looking for my assets in public/assets/styles and public/assets/scripts, which naturally 404s.
Here's what I did:
- 
Installed grunt-cli:
npm install grunt-cli -g - 
Added Half-Pipe to my Gemfile:
gem 'half-pipe', github: 'd-i/half-pipe' - 
Installed Half-Pipe:
bundle install rails g half_pipe:install - 
Installed Bower dependencies:
bower install - 
Moved assets to new location:
mv app/assets/javascripts/* app/scripts/ mv app/assets/stylesheets/* app/styles/ mkdir public/assets/ mv app/assets/images/ public/assets/ rm -rf app/assets - 
Replaced my CSS Sprockets directives with Sass imports
 - 
Wrapped the JS files referenced in
application.html.erbin:'use strict'; define('myapp', [ ... ], function($) { ... } - 
Then I wrapped the JS files referenced in the
defineabove in:define(['jquery'], function($) { ... }); - 
Removed
config.assetssettings fromconfig/application.rb - 
Manually added my libraries from
bower_componentstoconfig/build.js - 
Started the server (no errors):
grunt server