The Composer loader (as far as I can see from the interface) only allows to register a array with a complete map that's way Composer uses [0] but I can't get access to the ClassMapGenerator object instance from the outside.

[0] https://github.com/composer/composer/blob/090230519698ddb34eb710a16fa2c49fc719cea7/src/Composer/Autoload/ClassMapGenerator.php