There is a special string pattern to use base_path() into .env file variable ?
Something like : MY_PATH=%basePath%/my/path
You can use any external environment variables in the .env.
For example, you could set BASE_PATH in a shell call:
BASE_PATH=/Users/home php artisan my:command
Then you simply use them in the .env e.g.
MY_FILE="${BASE_PATH}/files/something.txt"
I think you can also assign environment variables in your web server.