Using Drupal as a server application user front-end (feasability question)
Absolutely.

Do not be put off by the 'CMS' label, Drupal is actually quite a framework in itself.  Drupal really does a good job for small to mid-sized applications, as the application gets larger though one has to tweak and push the limit a bit more. To be fair, this is true of most any PHP application. 

I have built quite a few applications myself that leverage the power of Drupal, some internal, some public: production management, calendaring applications, rental/item reservation for 4000 items +, mailing list management, (and more).

In my (very humble) opinion, Drupal saves a lot of time, especially for the 'user-management' aspect of application building. As a bonus, you have the rest of the framework to play with when developping your app.

Hope that helps, happy coding, good-luck!