IME, when you use an unlisted CPT code it gets kicked out automatically and you have to submit a letter and the op note.
I generally try to find the real code that most closely matches what I did, double the price and use that for my charge for the unlisted code. In the appeal letter to the ins co. explain how the procedure was similar to the 'real' code, and emphasis how much more work/harder it was.
The listed code acts as an know 'anchor' for your negotiations with the ins co and gives them an idea of what you did and you letter and op note explain what you really did and why it was some much harder and different that the 'real' code and why you should be paid more.
hope that helps.