Cleaning a rusted damascened piece is a very tricky task.
For very little rust over gold koftgari, I used Picreator's Metal Decorroder, then gently wiped off the rust. I managed to leave most of the koftgari in place but there were some losses.
For a more heavily corroded piece, it might be a completely diferent story and I would like to know the answer myself.
PS: What about some electric rust removal?!