I'm not a lawyer (though I sometimes like to pretend), but had the following thought:

If I book using a code such as 5760333, and National lets me take the car without asking for proof that I am entitled to use this code, could one not argue that by doing so, National agrees to let me pay the rate associated with that code and that I am entitled to any benefits (i.e. free LDW) that come with that code?

I am sure that there is some fine print somewhere in the rental contract that would nullify the above, but it is something to consider.