In my experience, none of the hotels in which I worked allowed anyone who wasn't connected to the Sales Department (Sales Manager, Reservations Manager, Revenue Manager) or a member of executive level management (General Manager, Assistant GM, Director of Operations, Director of Sales) to negotiate rates. If a Front Desk staff member or Night Auditor entered into pricing negotiations or offered discounts without authorization, it resulted in major disciplinary action.
Walk-ins got whatever rates where available on the computer. Those rates were identical to what the website and/or chain "800" number was selling because we were required to have rate parity with the chain. Offering lower rates could result in a negative mark in the hotel's annual review. Too many negatives does risk the franchise license, as arlflyer notes. (Honestly, it takes a LOT for that to happen, however.)
Personally, I think the likelihood of finding walk-in deals to be pretty slim. First, it is a violation of the franchise agreement. Second and much more importantly, if a traveler takes the time to stop and come into the hotel to inquire about a room, they're usually pretty price-insensitive. When practically everyone has the internet in their pocket, the physical act of entering the hotel tells the hotel the person probably wants to stay there. Why would they offer a discount to that type of customer?