Efficient way to keep Drupal 7 session alive from another web app?
If you're worried about unnecessary overhead, why not check out the High-performance JavaScript callback handler module. 

It allows you to control what exactly gets loaded during the Drupal bootstrapping process, so that you would only spin up the bare essentials to keep your session alive from another app.

Check out this blog post for a good breakdown of how you implement this module: 

High performance AJAX callbacks with Drupal 7 and the JS module