Using "drush make" to maintain the source tree for a Drupal site, and avoid killing kittens

To avoid killing kittens through hacking Drupal core I've been maintaining a source tree using Mercurial (see: How to use Mercurial to hack Drupal Core without killing kittens). But, I had an inspired idea recently to see how well "drush make" could take care of the job and maybe stop using a private source tree. One of the key required features would be to maintain a set of local patches. I'm pleased to report some positive success and it looks like a useful method for a D6 to D7 upgrade.