Seems like it could be a part of OSM. And indeed, there's "amenity=piano" [1], as well as "musical_instrument=piano", with overpass-turbo [2] even finding some pianos using the latter.
You could also implement OAuth (sign-in with OSM) to do edits, similar to how some tools like https://maproulette.org/ work, though you'll get less contributions if you don't have an option for anonymous public edits.