You're going to have to reinvent the wheel if you use a hook. Whatever you end up with will be a hack to get the roughly equivalent result of setting ENVIRONMENT.
If you don't want to use existing methods to set ENVIRONMENT, just find the define('ENVIRONMENT', ...) line in index.php (it should be the first line of executable code in the file) and add whatever logic works best for your development environment.
Another possibility would be to simply modify the default ENVIRONMENT in that line to 'production' (instead of 'development') and have your developers set the CI_ENV server variable in their development environments.
