Those supplies will still be rectifying and are probably flyback converters. I'm a little surprised they work that far down. Rewinding the transformer would be difficult since they are probably all encapsulated.

Edit: If you can get a China charger to work properly under load, then great.

As Hugo says, just rectify (half wave should be enough but a full bridge can small enough) and use a high efficiency switching buck convertor. Just need to watch for the input voltage rating since you'll have a fairly high output from the rectifier.