I would like to create a login method with a custom icon, but I am so far unable to figure out how I should do that.

To add an icon to a login mehtod, you create a file resources/sprite/login_1x/XYZ.png and add the following to the associated src/auth/PhabricatorAuthProviderXXX.php:

protected function getLoginIcon() {
  return 'XYZ';
}

Afterwards you have to call two scripts to create the sprite map:

./scripts/celerity/generate_sprites.php
./bin/celerity map

However, I provide my login method as a library in a different repository/directory (to be enabled via conf/local/local.json: load-libraries). How am I supposed to make the icon available in this case?