Can I control which database Drupal uses for certain data? It is of course possible in a custom modules to use two different databases (and I mean different databases not different RDBMSs!), but is there a way to use two different databases, and then force Drupal to keep certain data in one and other data in the other?

For instance, keep nodes (and closely related tables) in one, and all site configuration in another.  (Perhaps, while sorting it out, it might be useful to have a third as the default.  That way everything that's not sorted into either the content or the configuration database would remain in the default until you decided where it best fit.)

How can this be done, without major hacking of Drupal's core?
If it's not possible at the Drupal level, this might be possible at the MySQL level using the FEDERATED storage engine.  This is a thoroughly untested solution, however, and it might not play nice with Drupal.