gistfile1.rb

# TODO First try it like below and check if Xcode is happy with the result.
#      I don’t remember if the new config /has/ to be defined on the project-level as well.

# Add a duplicate of the target-level Release config to each target.
project.targets.each do |target|
  build_config = project.new(Xcodeproj::Project::XCBuildConfiguration)
  build_config.name = 'AdHoc'
  build_config.build_settings = target.build_settings('Release')
  target.build_configurations << build_config
end

This is untested code btw.