i think you are supposed to be able to mappable=false to achieve what you’re trying to do, but it seems to behave inconsistently for me. there’s also geo=false, which seems to also pick up observations with private locations, but it seems to behave more consistently – so that may be the best filter unless someone can find something better.
maybe the addition of geoprivacy=open might help exclude observations with private locations.
UPDATE: sorry, i think i read your original request backward… so geo=true should probably do what you’re looking for (to find observations that do have a location).