Just load the Rakefile:
==> foo.rb <==
require 'rubygems'
require 'rake'
load 'Rakefile'
Rake::Task[:some_task].invoke
==> Rakefile <==
task :some_task do
puts "some_task"
end
Rake::Application is all about command-line processing, default rakefiles, output, etc. You might not need any of that.