I am a singer and I would also like to know!
I know MH mentioned something about lobelia expelling mucous from the lungs (via coughing I believe) but I don't know if that would help with stuff on the vocal cords. Do you think it is stuff dripping from your head onto the cords, or just something "permanently" stuck there? I find that green tea has properties that stop mucous production and clear up my head for a while but sometimes too much leaves the mouth and throat too dry. I've heard that swallowing a teaspoon of olive oil is a good way to lubricate before singing but have yet to try it.
SO if anyone has any concrete info, please tell!