I'm not sure the overlapping pins idea would work for e.g. a 5 floor building with no multilevel maps and 6 businesses to a floor. Which is a common thing in some of the places Google maps.

Works for me. If I search "restaurants", and I see a building full of pins. I can now go to that building and look at all the restaurants.

You don't want to show every business as a default view, of course.