This came up a little while ago and I asked my professor at that time. He basically stated that based on the legal definition of fraud in common law, doing this would likely constitute fraud (he was talking about different things that have to be the case for fraud to have existed, but I don't remember all of them-3 that I do remember are: the statement/representation is false, the person has to know it to be false, and the hotel has to believe it is true). But he also said that he's never done it so he doesn't know for sure whether the person using the corporate code has to accept something. He said that technically the person may be able to argue that he wasn't aware the rate is only for employees.
He also said that even if it was fraud, practically speaking, the worst thing that would happen is they charge you full price for the room. I tried to get more details from him about possibilities but he just didn't know enough about the hotel business/reservation process, etc...

Note that this information is not from my knowledge. It is from someone else who I spoke with a little over a year ago so you can make your own judgments about whether my brain has screwed with my memories