Has she had a full vet work-up? If so then my next suggestion would be diet and look at yourself.
If this is causing you stress and negative emotion she will pick up on that---and pluck because you are feeding her energy/attention about it and because she feels you are stressed (and that somehow the flock is in danger) and it will make her anxious.
Also, if you're the kind of person constantly watching her or fretting stop that (I have a bad habit with that with Jacko who also plucks). Just stop. Pretend like its not even happening and you dont even care. Reward her when she plays or vocalizes or does something other than pluck.
I'd look at her diet and see if there are allergies or triggers. I'd also add Red Palm Oil to her diet (does wonders) and spray her with aloe vera--no doubt the picking has irritated her skin which only causes more plucking. The one thing that really, really helped Jacko was the aloe vera.
Good luck, and I think the important thing with our pluck ducks is to love them nekked or not
