One aspect to consider with these stories is that the simple nature of the DC3 external control locks is such that it is possible they may not make the surface totally immovable and airflow and vibration would also tend to reduce the severity of their grip. Another point is that the loading of the aircraft could have been fortuitous in that it had the aircraft close to trim with the elevator in the 'locked' position. I certainly would not suggest the stories were ludicrous.
Neither would I suggest it was appropriate to run some flight tests with the locks in to get more data.