It would be a technical violation of the law to reveal to ANYONE including your lawyer that you received an NSL.
"…prohibits you…from disclosing this letter, other than to…an attorney to obtain legal advice or legal assistance with respect to this letter.""
Source: http://upload.wikimedia.org/wikipedia/commons/9/91/EFF-IA_Na...
I can't see a reasonable way of establishing bona fides other than creating a strong implication/inference of "national security".
(My wife is a federal attorney, and it is disappointing how warped "the government's" perspectives are on rights and what is reasonable.)