Yes, actually I do. Gag orders aren't unique to the NSL and if your attorney believes the case is too complicated they will be able to help you find a specialist.
The FBI does in fact has some guidance that your attorney will be able to locate[1]
[1]: https://vault.fbi.gov/National%20Security%20Letters%20%28NSL...
The point being made upthread is that there is no black and white guidance, there is no secret NSL Guide Book for attornies.