I have a cURL PHP script which is able to validate a username/password against the external source.
What is the best way to integrate this as a login requirement for (select) users in Drupal?
The idea would be to add the external authentication as a login requirement for a role.
The preferred way would probably be to write a little module that implements hook_user for $op=="login"...that allows you to perform any logic you want to whenever a user attempts to log in.
Alternatively, could you set up LDAP? The Drupal LDAP module is pretty nice.