From chriseppstein:

Setting all the css locations for the Sass::Plugin.options[:template_location] hash to the same location can result in sass files of the same names but in different output directories overwriting each other. It's probably not too likely, but I'd recommend against it. Instead, I think you should be able to remove "#{RAILS_ROOT}/public" from each and replace it with "#{RAILS_ROOT}/tmp/sass-output"

Might pay to pass the RAILS_ROOT into the middleware in init.rb:

ActionController::Dispatcher.middleware.use SassOnHeroku, RAILS_ROOT

Then in a normal rackup, it can be used:

use SassOnHeroku, Dir.pwd

Maybe use Rack::File to serve the generated .css files?