We are using ISPConfig 3 to manage our VPS. We have created a site with a shell user assigned to that site. There is an application installed on the VPS that is called drush (shell application for managing drupal sites). The latest version (7-dev) of drush requires installation through the composer. It is what we have done for the root user following this tutorial. Drush is available and can be used by the root user as well as by any user that was created through the ssh (not ISPConfig's panel) on our droplet. Question: how can we make drush available for shell users that were created by ISPConfig's user interface???
When the shell user is jailed, then you will have to install drush with the jk_cp command of the jailkit package into the jail of the user.
If a user is not jailed, then he has full access to all system tools. So no further actions should be required to access system wide commands.
Can you please test that from your side? Drush is a widely used tool! It is sad that it is not working on the "Perfect Server" Here is a tutorial that we used to install latest version of the drush http://simpleinformation.com/blog/step-step-instructions-install-new-drush-drush-7-required-drupal-8
I have a perfect server system here and one of the clients on that server is using drush for his drupal website daily and it works fine. So please do not assume that something is not working in a setup just because it dont work on the server that you installed. Off course we can test that for you. You can contact the ISPConfig business support here http://support.projektfarm.com/ to hire one of our administrators to test that on your server.