Have module .install create a table in a non-default DB
To my mind, the entire purpose of hook_schema is to build the Drupal schema. If you need/want to create tables outside of Drupal, it seems like just using PDO in a hook_install() and hook_uninstall() implementation might be the way to go. Can you explain your use case a little bit more?

All that said, I don't see an obvious reason for your code to fail.