If you were to be in an accident involving any significant amount of damage, I would expect the rental agency to investigate whether you were actually traveling on business. If you were not, I think that they could void the LDW coverage, and you -- or your personal auto-insurance company -- would be on the hook. National might even decide to put you on their DNR (do not rent) list.

Bad idea. Don't do it.

Have you checked to see if you can get a low rate with some other codes you can legitimately use? If you are a college/university grad, have you checked to see whether you are entitled as an alumnus to use your alma mater's leisure code?