A few days ago, while I was writing a bit of Silex code and grumbling at Doctrine DBAL's lack of support for a SQL Merge operation, I wondered if it wouldn't be possible to use DBTNG without the rest of Drupal.
Obviously, although DBTNG is described as having been designed for standalone use:
DBTNG should be a stand-alone library with no external dependences other than PHP 5.2 and the PDO database library, in actual use, the Github DBTNG repo has seen no commit in the last 3 years, and the D8 version is still not a Drupal 8 "Component" (i.e. decoupled code), but still a plain library with Drupal dependencies. How would it fare on its own ? Let's give it a try...