Since I export under license from the US Fish and Wildlife department all the time, I don't expect to need to state anything once there is no ivory. The documents must be signed under penalty of perjury in regards the animal products contained, so a declaration on the invoice of something like "contains no ivory. All ivory colored materials are synthetic" should suffice.
That being said, we are providing the customer with photos of the ivory being removed, the synthetic ivory being inlaid, and a sample of the synthetic ivory for comparison/testing if needed.