The catchword in your post was the word "free."

Free only happens when a group feels that they can make up the expense by selling people on something during the free experience.

I love the idea of a layover tour, but I would scratch out the word "free" and look at options by local small businesses.

There is a huge number of "airport shuttle" companies. I suspect that most of them would be happy to give a tour. I have never been in a limousine. I understand that these companies are willing to charge lower rates for off peak use like a midday layover tour.

By using the small business approach, one is more likely to encounter local small businesses and connect with the lay over city in a more authentic way.

Going the small biz route also means that one can find a layover tour at every airport.