application.rb

class Application < Rails::Application
  # ...
  config.autoload_paths << File.join(config.root, "app", "workers")
  # ...
end

assets.rake

task :generate_appcache_manifest => :environment do
  # ...
  puts Rails.configuration.autoload_paths.inspect
  purger = VarnishPurgeWorker.new
  # ...
end

# app/workers/varnish_purge_worker.rb defines VarnishPurgeWorker

output.txt

% rake assets:generate_appcache_manifest --trace
** Invoke assets:generate_appcache_manifest (first_time)
** Invoke assets:environment (first_time)
** Execute assets:environment
** Execute assets:generate_appcache_manifest
["/var/www/myapp/app/decorators", "/var/www/myapp/app/workers", "/var/www/myapp/lib"]
rake aborted!
uninitialized constant VarnishPurgeWorker