I have often, and properly, taken advantage of negotiated local business rates when visiting clients. They'd say "tell them you're visiting us and ask them to give you our rate." I would.

Obviously, my employment with the client couldn't be verified because it didn't exist. I suppose they could have asked me for some indication that I was really visiting that company, but no hotel ever did, and I never thought to carry any sort of proof.

I also used a hotel rate for people visiting patients at a nearby hospital quite a few times in a two-month period. Over time the desk staff came to know who I was visiting and so on, but before then nobody ever asked me for the name of the patient or any other proof. (Fortunately, no need for expressions of sympathy here. It turned out well in the end.)

So, I think the risk in doing this is negligible. Whether or not it meets one's ethical criteria is another issue.

(In my experience, this sort of rate doesn't show up on hotel chain central booking engines. You have to phone or e-mail the hotel itself directly.)