Is this information valuable to marketers?

I have this idea floating around in my head. I want to analyze radio stations and figure out who advertises.

Since its radio, there is a lot of local businesses.

If I managed to build that, could I sell access to that information to local marketing agencies?

I imagine it like this "Lawyer X and Y are advertising for four months consistently in this area" with this information a marketing agency could go out to similar local Lawyers and try to get their business.

Im not sure if it makes sense. Its not even a lead list, since you still need to do the work to find similar businesses.

Im thinking its less competetive since its all locally focused.