How to keep multiple drupal websites up to date
You could write scripts to do this.  You'd probably want to tie in to the Drupal unit testing module as well.

I can't provide you with any actual examples, because I'm not sure how it would be done reliably.

In my opinion, you're looking at a business problem that can't be fixed with a programming solution.  If I were in your shoes, I would start charging customers an ongoing maintenance fee for upgrades.