Due to an appeals court decision, NSL gag orders have been challengable since 2008 -- and it's very simple to do.

But the thing is, companies rarely care to fight for their users.

In 2010, the FBI admitted that the gag order has been challenged only 4 times, with a 50% success rate.

http://www.wired.com/threatlevel/2012/05/nsl-challenges/

So yes, that would be a fine clause for companies to sign onto as well.