I'd like to throw out something from another angle....the technology angle.
National is a business. National has an IT department who creates and maintains their website.
If there is a restriction on a code that can be legally upheld, it would not be very difficult for National to have a warning show up when the code is entered stating that restriction. Something like "You must provide ID from such and such a company at the time of rental". They do it for coupons that need to be printed. Marriott does it if you use a AAA rate or a Senior citizen rate. It's basic web development. So I don't think we should feel too bad for National who has chosen not to build restrictions into its reservation system which competing companies and other travel companies have done for this exact reason. If they tell you there is an ID restriction they can stop you at the gate and you have no argument.
I firmly believe that if there were a restriction on the code then it is up to National to inform their customers of this restriction. Like it or not, information is out there on the internet so they should certainly expect that this information can be found and they should have a process in place to deal with it.
Before anyone makes an assumption, I traveled for a living for 9 years so I rented lots of cars and used lots of codes for cars and hotels and if I need ID I make sure I'd have proper ID. I don't remember if I sign the contract or not on the way out - I'd assume I'd signed something. I would love to see what the rental contract says to find out if there is anything in there about agreeing that the rate that I reserved was a rate I was eligible for. Does anyone have that?
Before the morality police steps in....I used the code that was 'complimentary' without ID needed for my Executive Emerald Aisle rental coming up in November. Without this rate, I would not have been able to fly for this trip so it is worth the potential aggrevation if the gatekeeper at TPA (where I am flying into) gives me trouble. There is absolutely nothing on my reservation indicating that I cannot use this and I used it in good faith as complementary means to me that it is extended as a courtesy. Some may not feel it is worth the aggrevation but for me it is. I have every intention of leaving that airport with an executive aisle car at the rate I reserved - with my reservation in hand showing no restrictions.