Hi explorador - I did solve the problem using the brute force method mentioned above. Thanks for asking though.
In my case, there wasn't really an "old site" in any meaningful sense. I was trying to get data from Drupal 6 + Gallery into just Drupal 7... so I was doing a lot of manual stuff on the DB. Never did figure out the actual source of the problem, but sometimes it's less time to cut your losses and go back to a more manual approach.