What I was thinking is that if
you add a user to drupal it could populate the ispconfig data.
There could be a drupal module that calls to the api for ispconfig.
The admin of drupal would configure the module to know which ispconfig client to configure.
For example web38_xxx
then when I add the drupal user testuser
It creates the web38_testuser in ispconfig and sets up the same password as the drupal user.
Then if I change or delete the user and or password it is updated in ispconfig as well.