It borders on imposable to get anything special. They tend to get hung up on the final dimension instead of listening to what you tell them to put inside.
I asked for very specific minimums on both copper and core that I need for medical isolation in a transformer and I never get it. They seem to understand statements about copper but never listen to core thickness statements.
My next revision I am going to have to specify the isolation ratings. CE requires 9mm between the line and the user and a 5KV hipot rating. I am not certain I am getting that now.
That said, if you can maintain the core thickness the impedance is not a problem using microstrips or striplines.